Oceans doi: 10.3390/oceans7040061

Authors: Ting Zhang Min Liu Zhenwei Su

The prediction of ocean wave heights and periods is crucial for reducing maritime accidents. The application of neural networks to predictive modeling eliminates the need for complex parameter schemes that describe specific physical processes, thereby reducing computational costs. This study presents two single and three fusion models based on neural-network architectures: Convolutional Neural Network (CNN), Long Short-Term Memory (LSTM), LSTM with an attention mechanism (LSTM+Attention), LSTM fused with a CNN (CNN+LSTM), and LSTM with Ensemble Empirical Mode Decomposition (EEMD+LSTM). The models were compared on multiple prediction lengths based on multivariate datasets from two oceanographic stations southeast of Fujian Province. All models performed well for prediction lengths of less than 24 h, with the coefficients of determination for wave height and period prediction higher than 0.7 and 0.6, respectively. For prediction lengths longer than 24 h, the predictive capability of EEMD+LSTM was superior to that of the other models. For a 48 h prediction length, the predictive results at each time step were compared with the true values across four typhoons. At the 24th and 48th time steps, phase lags were observed in the results from the CNN and LSTM. In contrast, EEMD+LSTM better captured the trends of the true values.

Oceans doi: 10.3390/oceans7040060

Authors: Hongxiao Yu Qiaoyan Cai Jianqiang Zhang

To address the lack of compliance with the International Regulations for Preventing Collisions at Sea (COLREGs) in conventional timed elastic band (TEB)—based dynamic path planning for autonomous surface vessels (ASVs), this study proposes an intelligent path-planning framework that integrates an improved jump point search (JPS) algorithm with a COLREGs-aware TEB approach. At the global planning layer, an enhanced JPS algorithm incorporating redundant-node elimination and cubic Bezier curve smoothing is developed to construct the JPS–PB method, thereby reducing path redundancy and improving trajectory continuity. At the local planning layer, COLREGs encounter rules are reformulated as differentiable cost functions and embedded into the TEB optimization framework, enabling rule-compliant collision avoidance in typical maritime encounter scenarios, including head-on, crossing, and overtaking situations. Simulation results demonstrate that, compared with the conventional JPS algorithm, the proposed JPS-PB method reduces the path length by up to 8.6%, decreases the cumulative steering angle by 58.6%, and lowers the maximum steering angle by 88.6% on a 12 × 12 grid environment. Furthermore, compared with the conventional TEB algorithm, the proposed COLREGs–aware TEB (CTEB) increases the Distance at the Closest Point of Approach (DCPA) by 543%, 20.5%, 4.8%, and 13.8% in starboard-crossing encounters, head-on, overtaking, and special port-side-crossing encounter scenarios, respectively. Parameter sensitivity analyses further indicate that CTEB maintains stable collision-avoidance performance and rule-compliance capability under variations in the danger–distance threshold, danger–time threshold, and COLREGs cost weight. The results verify that the proposed JPS–PB+CTEB hybrid framework effectively improves path quality, dynamic obstacle-avoidance safety, and navigation-rule compliance for autonomous ASV operations.

Oceans doi: 10.3390/oceans7040059

Authors: Gonzalo S. Saldías Richard Muñoz Alexander Galán Roberto Aedo-Garcia Carlos Lara Fabián J. Tapia

Satellite chlorophyll–a (Chla) products are widely used to study coastal productivity, but their performance often degrades in river-influenced and optically complex waters. We evaluated MODIS-Aqua Chla retrievals in the coastal upwelling area off central-southern Chile, a region strongly affected by seasonal river plumes, using monthly in situ Chla and hydrographic observations from Station 18 (August 2002 to September 2011), daily MODIS products, and matchup analyses based on 3 × 3 pixel windows and 1-, 3-, 5-, and 7-day composites. MODIS Chla and normalized Fluorescence Line Height (nFLH) reproduced the broad seasonal cycle, with maxima during spring–summer, but default MODIS Chla systematically exceeded in situ observations, particularly during periods of enhanced turbidity and river-influenced optical complexity. Among the raw satellite products, 1-day MODIS Chla matchups showed the strongest agreement with in situ Chla (r = 0.77, RMSE = 8.5 mg m−3), whereas 5-day composites increased matchup availability to 95% but reduced the correlation (r = 0.46, RMSE = 10.5 mg m−3). In contrast, nFLH showed more stable performance across composite lengths, although it underestimated high Chla values and should therefore be interpreted as a complementary fluorescence-based diagnostic rather than as a direct substitute for locally validated Chla retrievals. A gradient boosting model trained with MODIS remote-sensing reflectances improved the correspondence between satellite and in situ Chla relative to the default MODIS product within the available Station 18 matchup dataset. Because this model was evaluated using cross-validation rather than an independent regional validation dataset, the machine-learning results should be interpreted as a local proof of concept rather than a fully validated regional algorithm. These results indicate that standard MODIS algorithms overestimate Chla in this river-influenced upwelling system and highlight the value of local correction approaches, including machine-learning methods, for improving coastal ocean color products, provided that future applications include independent spatially distributed validation and improved bio-optical characterization of river-influenced waters.

Oceans doi: 10.3390/oceans7040058

Authors: Fatima Abrantes Sandra Gomes Marta Salvado Vitor Magalhães Emilia Salgueiro Teresa Drago Livia Cordeiro Filipa Naughton

This work presents new data and several age–depth reconstructions based on distinct approaches to improve previously published age models for sedimentary sequences collected on the Iberian Margin middle shelf: PO287 6-1B, 2G (Porto); PO287 26-1B, 3G, and D13902 (Lisbon); and POPEI VC2B (Algarve). The new age models are constructed using radiocarbon dates calibrated with the IntCal20 calibration curve and smooth-spline regressions from the CLAM (non-Bayesian) and Bacon (Bayesian) models. A comparison of the age–depth models generated by the different approaches shows that, beyond the distinct solutions produced by CLAM and Bacon for each site, the differences between the two methods are inconsistent across sites. Furthermore, neither model yielded reliable results for the discontinuous sedimentary sequence with reworked older sediments and a hiatus. In this specific case, the “classical” linear regression (best-fit) approach appears to yield the results that enable intercomparison across all sequences. This exercise confirms the record of the 1755 earthquake and tsunami in the Lisbon core splice and a potential record of the same event in the Eastern Algarve, although the collected data indicate a more distal or lower-energy wave deposit.

Oceans doi: 10.3390/oceans7040057

Authors: Emeline Cadier Claire Maraldi François Bignalet-Cazalet Nicolas Cuvillon Geoffroy Bracher François Boy Bastien Courcol Cécile Kocha Victor Quet Franck Octau Pierre Prandi Aurélien Deniau Cyril Germineaud

Over the past three decades, five satellites have succeeded one another on the reference orbit, building the longest continuous climate record of global sea level measurements. Its continuity is ensured thanks to tandem flights between consecutive satellites. In this paper, we demonstrate that the first satellite of the Sentinel-6 series (Sentinel-6 Michael Freilich) has enabled the detection of a processing anomaly in the Jason-1/2/3 ground segment. An inconsistency in the altimeter range reconstruction has been identified, causing its underestimation by 3.65 mm. At certain latitudes, determined by the satellite’s orbital velocity, the altimeter range shows no effect from the anomaly. For the reference orbit, the range is not affected at the poles, around the equator and, for ascending tracks, at 40° S. All Jason Geophysical Data Record (GDR) versions prior to GDR-G are impacted by the described processing anomaly. While a full reprocessing of the Jason data with the GDR-G standard is pending, this paper presents a latitudinal empirical correction to be applied to Jason datasets generated with GDR-F and earlier ground segments. This correction, to be applied on the altimeter range, is derived from one month of patched Jason-3 data and is intended for reference orbit only. Additionally, SWOT Nadir ground processing is also affected by the same processing error and has been corrected from the GDR-S2 version onward. Finally, our analysis shows a negligible impact of this processing anomaly on Jason Level-2-derived products, models and metrics.

Oceans doi: 10.3390/oceans7040056

Authors: Nick Z. Zacharis Kyriakos N. Sgarbas George Leventakis Petros Savvidis Dimitris Papachristos Nikitas Nikitakos Iosif Progoulakis

This study outlines a comprehensive analytical methodology applied to a vessel’s operational data to establish a rigorously validated, data-driven baseline for the Marine and Health Usage Monitoring System (MHUMS) algorithmic modules. By correlating a continuous three-month dataset of sensor telemetry with historical alarm logs, the primary objective was to develop and train a robust predictive model. This study addresses the critical challenge of converting continuous marine sensor telemetry into actionable early warnings by establishing a validated predictive baseline. The proposed baseline predictive architecture achieved a Recall of 0.924 and an area under the ROC curve (AUC) of 0.939 for a 15 min forward prediction window, proving the feasibility of transitioning from reactive diagnostic monitoring to proactive early warnings.

Oceans doi: 10.3390/oceans7040055

Authors: Robert L. Courtney Danica Lennox-Bulow Tobin D. Northfield Jamie E. Seymour

Carukia barnesi is a small tropical Cubozoan that causes painful and potentially fatal stings (Irukandji syndrome) along the north-eastern coast of Australia during the summer monsoonal months between November and May. This research establishes the thermal and osmotic tolerance of the medusa stage of the Irukandji jellyfish C. barnesi to determine if environmental parameters influence the marked seasonality of this species. By quantifying oxygen consumption over a range of temperatures, the theoretical minimum thermal requirement for the long-term persistence of C. barnesi medusae (i.e., theoretical zero) was estimated at 21.5 °C, which does not explain the seasonal occurrence of this species. The highest bell contraction frequency was determined to fall between 27.5 °C and 30.9 °C, which encompassed the typical thermal regime in situ. This research concludes that the deleterious effects of reduced performance associated with a departure from suitable environmental temperatures may better describe the seasonal pattern of this species than the theoretical thermal limits. Conversely, departure from suitable thermal regimes did not explain the southern distribution limits of this species, suggesting that C. barnesi could theoretically persist further south than their loosely defined southern distribution limits. Salinity also had a significant effect on C. barnesi medusae whereby the highest bell contraction frequency occurred at salinity levels above 33.6‰. Salinity levels below 29‰ reduced swimming capacity and became lethal below 25‰. This result supports the hypothesis that C. barnesi medusae are oceanic and/or neritic and cannot persist in estuarine environments where low salinity conditions commonly occur. Furthermore, the respiration rate of C. barnesi was lower at night, providing further evidence that this species is less active during night conditions, presumably to conserve energy.

Oceans doi: 10.3390/oceans7040054

Authors: Bao Wang Jie Xiao Chuhan Feng Xishan Pan Bin Wang

Accurate prediction of significant wave height (SWH) is essential for fisheries management, coastal socioeconomic activities, and marine ecological conservation. In recent years, deep learning-based bias correction has shown considerable potential for improving numerical wave forecasts. However, many existing approaches are still constrained by limited receptive fields and often struggle to capture long-range spatiotemporal dependencies in wave forecast errors. To deal with this issue, we adapt and improve a video prediction framework, namely the Vision Mamba Recurrent Neural Network (VMRNN), to model and correct the spatiotemporal patterns of SWH prediction biases. Comprehensive evaluations show that the multi-channel VMRNN achieves consistently high predictive accuracy across different forecast lead times and sea-state conditions. When validated against reanalysis data, the proposed model reduces the root mean square error (RMSE) of WAVEWATCH III forecasts by 28.2%, 26.1%, and 24.7% at lead times of 24, 48, and 72 h, respectively. It also preserves the spatial structure of SWH fields quite well, with the spatial structural similarity index remaining as high as 0.945 even at the 72 h lead time. Regional assessments over high-wave areas further indicate that VMRNN can effectively reduce both the mean error and the systematic overestimation commonly found in numerical wave models. Additional validation using in situ buoys observations confirms that the model has a robust ability to correct systematic positive biases, especially for wave heights ranging from 0.5 m to 2 m. Taken together, these results suggest that VMRNN has strong spatiotemporal modeling capability and can serve as a promising post-processing framework for improving operational physics-based wave forecasting systems.

Oceans doi: 10.3390/oceans7040053

Authors: Tongtong Qie Jianyong Zheng Jianzheng Zhang Hongyu Wei Haolin Yang Kun Wei

Predicting a ship’s motion trend in waves is crucial for safe navigation and operation. Existing prediction models are mostly based on the assumption of local linear dynamics, which can achieve great performance in idealized ocean environments. However, ships typically sail in real marine environments with regular or irregular waves, which makes the robustness and real-time performance of ship motion estimation models particularly important. To address this limitation, this paper proposes a global linear predictor (GLP) based on the Koopman operator, which can effectively represent the nonlinear rolling dynamics of ships. Furthermore, the GLP model is used to predict and control the rolling motion of a ship in real time. The proposed method is validated in both regular and irregular wave environments. The simulation experiment results show that the accuracy of the proposed method is about 14% higher than that of other classical methods on ships’ rolling dynamics. And it achieves a more than 91% rolling reduction efficiency in all wave conditions, significantly decreasing the amplitude of a ship’s rolling.

Oceans doi: 10.3390/oceans7040052

Authors: Youssef Fannassi Younes Oubaki Zhour Ennouali Titus Karderic Williams Aicha Benmohammadi Ali Masria

Coastal zones are facing rising exposure to climate-related hazards alongside intensifying human pressures, which highlights the need for robust tools to assess vulnerability. This study uses a GIS-based Coastal Vulnerability Index (CVI) to quantify and map relative vulnerability along ~13 km of shoreline in Al Hoceima Bay (northern Morocco). The proposed CVI integrates eight geological and physical indicators, including geomorphology, shoreline erosion and accretion rates, coastal slope, elevation, natural habitats, relative sea-level rise, significant wave height, and tidal range. Spatial analyses were performed using remote sensing data, historical records, field measurements, and Geographic Information Systems (GIS). The analysis reveals that 37% of the shoreline is categorized as high vulnerability, 44% is moderate, and 19% is low. Highly vulnerable sectors are primarily associated with low elevations, gentle coastal slopes, sandy beach systems, limited natural habitat protection, and proximity to river mouths. These findings demonstrate that the applied CVI provides a rapid and cost-effective framework for identifying priority areas for coastal management and climate adaptation. The proposed approach offers valuable decision-support insights for sustainable coastal planning in Al Hoceima Bay and other Mediterranean coastal environments characterized by limited data availability.

Oceans doi: 10.3390/oceans7030051

Authors: Haruka Suzuki Tomoya Aoki Masakazu N. Aoki

To determine the factors influencing juvenile recruitment of the kelp Eisenia bicyclis, a seven-year monitoring survey was conducted in an area affected by seismic subsidence caused by the 2011 earthquake and subsequent breakwater restoration. Juvenile recruitment was high in September 2011 and in May to June of 2013–2015, but low in 2012, 2016 and 2017. Analysis of the relationship between environmental factors and juvenile recruitment revealed that recruitment was associated with light intensity, with lower water temperature two months prior, and an increase in nutrients four months prior. The seasonal increase in nutrient concentrations during winter may have been influenced by the seasonal northwestward coastal current. In contrast, despite elevated nutrient concentrations, recruitment was relatively poor in 2016–2017. This might be attributed to the unstable seabed environment associated with the breakwater construction. Our monthly monitoring of both the number of E. bicyclis juvenile recruitments and environmental factors at the same site demonstrated that a time-lagged increase in nutrient concentrations and a decrease in water temperature are associated with Eisenia bicyclis juvenile recruitment. This study provides fundamental information on kelp recruitment that will contribute to predicting the recovery of kelp communities following disturbances and recruitment dynamics under environmental change.

Oceans doi: 10.3390/oceans7030050

Authors: Denise Sodré Aurycéia Costa Elton Silva Luci Pereira Rauquírio Costa

The Amazon coastal zone exhibits remarkable habitat diversity and species richness, with nutrient-rich estuaries playing a crucial role in local food webs and supporting fish and other aquatic organisms. To examine the distribution of fish larvae and juveniles in the Taperaçu Estuary and their relationship with environmental variables, monthly sampling was conducted at two fixed stations in 2008. Samples were collected during flood and ebb spring tides using 500 μm mesh nets. In situ measurements of salinity, temperature, and dissolved oxygen were recorded, while pH and turbidity were determined in the laboratory. Abiotic variables did not differ significantly between tides, but salinity and dissolved oxygen were higher during the dry season. A total of 5175 individuals were identified, representing 17 families and 37 species. The ichthyoplankton community was dominated by Rhinosardinia amazonica, Anchovia clupeoides, Stellifer stellifer, and Microgobius meeki. Stations 1 and 2 showed differing abundance ranges, with higher values at station 1 during the rainy season. Preflexion stages were abundant at both stations, indicating the estuary’s importance as a nursery and development area for several fish species. Multivariate analyses revealed spatial and seasonal structuring of larval assemblages along the estuarine gradient, driven primarily by salinity, temperature, and turbidity. Our results emphasize the role of upper estuary sectors of eastern Amazonia as areas of spawning, larval development, and subsequent juvenile settlement, contributing to the dispersal of fish species throughout the estuary and adjacent coastal environments. The present findings also reinforce the ecological value of the studied Extractive Reserve and other protected areas along the Amazon littoral as essential habitats for larval refuge and development. The need for continued monitoring and preservation of these protected zones is evident.

Oceans doi: 10.3390/oceans7030049

Authors: Xiang Wan Lei Zhang Maolin Li

Accurately capturing the Kuroshio’s main axis and its multiscale frontal variations remains challenging due to the constraints of traditional fixed-section extraction methods. Here, we develop an adaptive iterative tracking algorithm utilizing high-resolution reanalysis data (2002–2024) that dynamically adjusts search directions and cross-sections via local velocity vectors, integrated with a dynamic step size and two-dimensional validation. Applying a multiscale variability decomposition framework across four key regions reveals distinct spatiotemporal dynamics. The North Equatorial Current bifurcation zone exhibits a significant strengthening trend driven by seasonal zonal and decadal meridional flows. Conversely, the Kuroshio east of Taiwan is dominated by high-frequency mesoscale processes (~70%) with a semi-annual cycle and no long-term trend. The East China Sea front maintains a highly stable seasonal meridional signal (25%). Crucially, the Luzon Strait intrusion shows a significant long-term weakening trend (~0.0029 m·s−1·a−1, p < 0.01), characterized by eastward strengthening and northward weakening, with ENSO significantly modulating its seasonal cycle. This approach substantially reduces systematic extraction errors compared to traditional fixed-section methods, as independently verified using satellite SST frontal gradients (median deviation < 0.2°), providing critical observational evidence for understanding western boundary current–marginal sea interactions and their dynamical responses under global warming.

Oceans doi: 10.3390/oceans7030048

Authors: Jihong Sun Guohui Su Huairong Song Jingpeng Liu Wenrong Lin Qi Xu Zonghua Liu

Wetland landscape classification is fundamental to monitoring changes in ecosystem patterns. This study proposes an ensemble classification approach that integrates Maximum Likelihood Classification (MLC) and Decision Tree (DT) methods with optimized feature selection for long-term wetland monitoring in the Liaohe Delta, China. Based on four periods of Landsat remote sensing images from 1987, 1997, 2007, and 2017, multi-dimensional features including PCA1, TCT, NDVI, and MNDWI were extracted to construct a hierarchical classification system comprising 13 landscape types. The results show that the integrated method achieved an overall classification accuracy of 87.71% and a Kappa coefficient of 0.85, improving by 16.50% and 19.72%, respectively, compared with the single MLC approach. The classification results reveal significant spatiotemporal variations in landscape patterns. Reed wetlands decreased from 1284.44 km2 in 1987 to 1006.70 km2 in 1997, followed by a recovery to 1275.53 km2 in 2017. In contrast, Suaeda communities experienced severe degradation, declining sharply from 227.48 km2 in 1987 to 30.52 km2 in 2017. Meanwhile, the coastline advanced landward by 263.24 km2, with the proportion of artificial shoreline increasing from 12.4% to 38.7%. The changes in wetland landscape types in the Liaohe Delta from 1987 to 2017 were mainly influenced by urbanization processes and ecological restoration policies. These findings indicate that the proposed method can effectively support long-term wetland landscape dynamics analysis and provide a useful reference for coastal wetland management.

Oceans doi: 10.3390/oceans7030047

Authors: Erik Coria-Monter Adolfo Gracia Elizabeth Durán-Campos César Flores-Coto Zayra López-Cabello Magali Roa-Venicio Alejandra Itzel Domínguez-Tavera

Holoplanktonic mollusks (Mollusca: Gastropoda: Pteropoda) are vital structural and functional components of marine zooplankton communities, characterized by high sensitivity to physicochemical shifts in the water column. Consequently, multidisciplinary assessments are essential to elucidate their community dynamics. This study investigated the epipelagic pteropod community structure in the deep-water basin of the southern Gulf of Mexico (sGoM) in relation to hydrographic features and circulation patterns. During the summer (September) of 2016, we collected high-resolution hydrographic data and zooplankton samples using CTD casts and oblique bongo net tows. Hydrographic data revealed intense temperature and density gradients, including a cold-dense core associated with a well-defined cyclonic eddy. The pteropod assemblage comprised 25 species from 13 genera and 10 families. Heliconoides inflatus (947.5 ind 100 m−3) and Limacina trochiformis (396.8 ind 100 m−3) were the most abundant species, whereas Cavolinia gibbosa (0.4 ind 100 m−3) and Cymbulia sp. (0.3 ind 100 m−3) were the least abundant. Horizontal distribution analyses revealed that the peak population densities occurred within the influence of the cyclonic eddy, particularly at its periphery where strong currents (>0.5 m/s) were recorded. A Canonical Correspondence Analysis identified temperature and salinity as the primary environmental drivers of community variability, while current systems significantly influenced the horizontal distribution of key species. Although pteropod research in the sGoM spans decades, most studies have been limited to taxonomic checklists, often overlooking environmental drivers and hydrographic influences. By applying a multidisciplinary approach to examine physical–biological coupling, this study advances the ecological understanding of this group within the historically underrepresented deep-water basin of the sGoM.

Oceans doi: 10.3390/oceans7030046

Authors: Tianchang Cui Hiroto Abe Tetsuya Takatsu Kenshi Kuma Yoshihiko Kamei Naoto Kobayashi Takahiro Iida Atsushi Ooki

We investigated the autumn-to-winter evolution of water-mass structure and nutrient concentrations in Funka Bay, southwestern Hokkaido, Japan, from October to February (2012–2019). Hydrographic and biogeochemical profiles show a recurrent seasonal transition from strongly stratified conditions in October, with low surface nutrients and bottom enrichment, to increasingly homogeneous distributions by mid-winter as vertical mixing intensifies. Depth-averaged nutrient concentrations generally decreased from October to December and increased from December to February, except during December 2015–February 2016. To assess whether February nutrient levels can be explained by Oyashio supply alone, we calculated February nutrient concentrations using a two-endmember mixing model (Oyashio endmember and December Funka Bay water) with an additional regeneration term that assumes nutrients consumed during the October–December autumn bloom were fully regenerated during December–February and redistributed by winter mixing. Under this framework, the expected February concentrations agreed with observations in all winters except 2015, when observed nutrients were lower than expected nutrients, consistent with additional biological drawdown after the early onset of the bloom by late February. These results indicate that the pre-bloom winter nutrient environment in Funka Bay is shaped by variable Oyashio intrusion superimposed on seasonal mixing and internal regeneration processes.

Oceans doi: 10.3390/oceans7030045

Authors: Haopeng Fan Shuling Xie Shuqiang Xue

The sound speed profile (SSP) is a core environmental parameter for underwater acoustic detection, navigation, communication, and other applications. However, its accurate acquisition is constrained by the sparsity of observational data and the ill-posed nature of inversion problems. This paper systematically reviews the research progress of SSP inversion under sparse observation constraints. The review traces the technical evolution from early physical models to current intelligent paradigms, classifies and compares mainstream inversion methods, presents typical application scenarios with quantitative case studies, provides a comparison of all kinds of SSP acquisition routes, and discusses critical challenges and future trends. The review reveals that current AI-driven methods achieve a practical accuracy of approximately 1–2 m/s but face bottlenecks in interpretability, cross-regional generalization, and extreme-condition robustness. Fusing physical constraints with multi-source sparse data (remote sensing, in-situ discrete measurements) emerges as the core direction for balancing inversion accuracy, efficiency, and cost. This paper provides a comprehensive reference for technical selection in marine acoustics, ocean observation, and underwater operations.

Oceans doi: 10.3390/oceans7030044

Authors: George Kokosalakis Xakousti Afroditi Merika Theodore Syriopoulos

Port State Control (PSC) inspections play a critical role in enforcing international maritime safety and environmental standards, yet little is known about how compliance behaviour interacts with economic cycles. This study examines the relationship between vessel detentions and freight market conditions using monthly data from the Paris and Tokyo Memoranda of Understanding (MoUs) over the period 2010–2021. A system of simultaneous equations is estimated using the Generalized Method of Moments (GMM) and Three-Stage Least Squares (3SLS) to address the bidirectional relationship between detention activity and freight market conditions, proxied by the Baltic Dry Index (BDI) and, for tanker specifications, the Baltic Dirty Tanker Index (BDTI). The results are consistent with a positive and statistically significant bidirectional relationship: vessel detentions increase during periods of strong freight market conditions, while past detentions are positively associated with freight rates, a pattern consistent with a signalling and sentiment channel. Institutional factors, including flag state quality, classification society affiliation, and ISM-related deficiencies, are also found to significantly influence detention risk, though their direction and magnitude vary across MoUs and vessel segments. These findings are consistent with the presence of opportunistic incentives during economic upswings, challenging the conventional expectation that stronger market conditions promote higher compliance. The study contributes to the literature by linking regulatory compliance with economic cycles and highlighting the importance of adaptive, risk-based enforcement strategies. It is important to note, however, that the aggregate nature of the data does not permit direct identification of firm-level behavioural mechanisms, and the findings should be interpreted as associational evidence consistent with these theoretical mechanisms.

Oceans doi: 10.3390/oceans7030043

Authors: Chih-Wei Chang Shiau-Yun Lu Chun-Pei Liao Wen-Yan Chiau Yi-Che Shih

Marine oil spills pose critical challenges to environmental sustainability and socioeconomic stability. Taking four pivotal cases as the entry point, this study uses comparative case analysis, semi-structured stakeholder interviews, policy analysis and international gap comparison to systematically analyze the evolution of marine oil spill governance in the Taiwan region of China over two decades, aiming to identify systemic gaps and propose actionable reforms. By integrating and explicitly detailing these multiple methodologies, this research not only identifies but also systematically examines the Taiwan region of China’s unique challenges as a non-UN-member entity navigating international conventions like the international convention for the prevention of pollution from ships, 1973, as modified by the protocol of 1978 relating thereto (MARPOL 73/78). Key findings reveal persistent issues in decision-support tools, fragmented inter-agency coordination, and legal inadequacies in compensation mechanisms. The study’s novelty lies in its rigorous synthesis of localized case-driven insights compared with global best practices, proposing a concrete, phased model for a unified task force and context-aware, data-driven contingency plans to enhance real-time response efficiency. It further advocates for pragmatic steps to align the Taiwan region of China’s Marine Pollution Control Act with international standards while critically addressing the transboundary collaboration barriers imposed by its political status, exploring potential pathways through sub-national and regional partnerships. Notably, the 2023 Angel Container case underscores the urgency of modernizing enforcement capacities and integrating advanced technologies. By bridging gaps in governance, legal accountability, and practical international engagement, this research not only advances the Taiwan region of China’s preparedness but also offers a nuanced and adaptable blueprint for coastal regions facing similar geopolitical and environmental constraints. Its recommendations hold significant implications for global marine pollution management, emphasizing the interplay of policy innovation, technological adoption, and pragmatic cross-jurisdictional cooperation.

Oceans doi: 10.3390/oceans7030042

Authors: Cristina Misic Anabella Covazzi-Harriague

Marine benthic ecosystems along urbanised coastlines face heightened vulnerability due to the cumulative effects of chronic anthropogenic stressors. Climate change intensifies these pressures through more frequent and severe storms, while ongoing coastal development adds further stress through infrastructure projects. This study examined how soft-bottom communities in the coastal NW Mediterranean responded to two major disturbances: an exceptional storm in 2018 and the construction of a new wastewater pipeline in 2019. Sediment grain size, organic content, bacterial abundance and enzymatic activity, and metazoan communities were analysed during summer of 2018, 2019 and 2020 and in the following spring period. Hydrodynamic forcing caused a general increase in the grain size in 2019. Meiobenthos responded with a strong decline in abundance, especially crustaceans, while macrobenthos changed from a mixed deposit-feeder community to a suspension-feeder dominated one. In 2020, the improved trophic value of sediment organic matter in the pipe area favoured bacterial increase. While meiobenthos abundance slowly recovered, the differentiation increased due to macrobenthic juveniles, resulting from increased macrobenthic abundance and diversity (mainly pure deposit-feeders). A clear shift towards organic enrichment-tolerant taxa due to wastewater release was not observed, given the contemporary presence of very sensitive organisms, indicating that co-occurring stressors can lead to nonlinear responses of the communities.:

Oceans doi: 10.3390/oceans7030041

Authors: Matteo Battiata Benedetto Sirchia Sabrina Lo Brutto

The armless snake eel, Dalophis imberbis, is a fossorial rare species. It is considered to be a non-target fishery resource with elusive behavior, and there is a paucity of knowledge regarding its distribution and biology. This study reports three new documented occurrence records of D. imberbis along the northern and southeastern coastal areas of Sicily (central Mediterranean Sea) during 2025. Specimens were collected at depths ranging from 43 m to an unusually shallow depth of 5.4 m, extending the known upper vertical limit of the species, which was previously considered a 20 m depth. Environmental parameters were collected through a multiparametric probe and integrated with products from the Copernicus Marine Service (CMS), providing new insights which highlight the presence of the species in relatively warm (17.6–20.8 °C) and moderately oxygen-undersaturated (6.9–8.5 mg/L) waters. A global distributional analysis was performed by aggregating the field data with literature records and datasets published in the Global Biodiversity Information Facility (GBIF), refining the distribution of the species in the Mediterranean and Atlantic Ocean. Thus, the three new records expand the known distribution of the species in the center of the Mediterranean Sea, providing an updated bathymetric range and the first preliminary insights into the environmental preferences of this data-deficient ophichthid. This work underscores the importance of combining traditional surveys with big-data repositories and remote sensing to monitor rare marine biodiversity.

Oceans doi: 10.3390/oceans7030039

Authors: David J. Marshall Natasha Khairul Naziratul Syaziyah Yahya Norazimah Duraman James R. Jennings

The dynamics of calcium carbonate structures in marine organisms (skeletons and shells) has become increasingly important due to heightened interest in marine environmental acidification. Research into molluscan shell corrosion and calcification in response to acidification is typically carried out in laboratory-controlled settings, which often overlooks the intricate interactions found in natural environments. Mollusks inhabiting intertidal zones are especially susceptible to intense shell weathering caused by tidal cycles of heating, cooling, wetting, and drying, exacerbated by solar radiation during periods of air exposure. We investigated the effect of sun exposure (solar radiative heating) on both outer shell corrosion and inner shell compensatory calcification in the tropical oyster, Saccostrea scyphophilla. Shell properties were compared between oysters from neighboring populations in sun-exposed and shaded habitats. Habitat temperatures were measured using iButtons, and right shell valve corrosion was quantified. Compensatory calcification was assessed through measurements of shell thickness, shell density, shell compression strength, and mineralogical properties. Our results revealed that oysters in the sun that experience global irradiance, higher temperature peaks and broader daily temperature ranges (averaging an increase of 10 °C) show considerably greater outer shell surface corrosion (87%) compared to shaded oysters (31%) that experience only diffuse irradiance. Sun-exposed shells also become thickened in the midsection and around the adductor muscle, and they are slightly stronger, indicating compensation for the outer shell loss. These findings highlight the need for caution when interpreting molluscan shell dynamics based on laboratory marine acidification protocols that fail to account for the many natural environmental factors influencing shell formation and dissolution.

Oceans doi: 10.3390/oceans7030040

Authors: Qian Fang Xiaoyu Yu Peng Wang

Langmuir turbulence is a key and common process in the ocean surface boundary layer, playing a major role in vertical mixing, heat flux, and material transport. However, because direct simulation of Langmuir turbulence demands considerable computational resources, parameterizations within established schemes like the K-profile parameterization (KPP) offer a practical alternative for representing its effects in ocean and climate models. However, Langmuir turbulence parameterizations based on KPP may overestimate vertical mixing when convection is significant. To address this, we introduce a dynamic weighting factor, based on characteristic velocity scales, to balance the contributions of convective and Langmuir turbulence. The improved scheme shows a significant enhancement in performance, especially under strong convective conditions. We compare and evaluate the new parameterization schemes against other widely used schemes in three typical scenarios. Additionally, we validate it using large-eddy simulation results and field observation data. Our enhanced mixing scheme is highly competitive and performs robustly under a variety of conditions.

Oceans doi: 10.3390/oceans7030038

Authors: Atiqur Rahman Sunny Md. Shishir Bhuyian Sharif Ahmed Sazzad Md. Faruque Miah Md. Ashrafuzzaman Kamrul Islam Md. Abdullah Al Mamun Shamsul Haque Prodhan

The present study examined fish biodiversity, livelihood dependence, cultural importance, and genetic connectivity in two ecologically linked habitats of the Sylhet region, Bangladesh: Hakaluki Haor and the Surma River. Surveys documented 60 fish species with distinct assemblage patterns between sites. Hakaluki Haor was dominated by floodplain spawners and small indigenous species that contribute to year-round subsistence harvests, whereas the Surma River supported a greater proportion of migratory and pelagic species, most notably Tenualosa ilisha. These ecological contrasts reflected differences in hydrology, habitat diversity, and fishing intensity. Household surveys confirmed the central role of fisheries in sustaining income and food security, while cultural practices surrounding hilsa consumption reinforced local stewardship norms. Mitochondrial cytochrome b sequence analysis of T. ilisha revealed low genetic differentiation between sites, indicating a single, well-connected stock maintained by seasonal flooding and the absence of major migration barriers. This convergence of ecological and genetic evidence supports treating the two sites as an integrated management unit. Effective conservation will require protecting hydrological connectivity, safeguarding dry season refugia, coordinating seasonal fishing restrictions across habitats, and incorporating cultural values into policy frameworks. The findings strengthen the scientific basis for national and regional conservation strategies and demonstrate the value of combining biological, socio-economic, and cultural dimensions in managing connected wetland–river systems. This approach can serve as a transferable model for other tropical floodplain–river complexes facing similar ecological and livelihood challenges.

Oceans doi: 10.3390/oceans7030037

Authors: Dimitris Klaoudatos Stefanos Gkourtsoulis Dimitris Pafras Alexandros Theocharis

The Mediterranean Sea is both a global biodiversity hotspot and the world’s most heavily invaded marine region, where non-indigenous species arrivals are accelerating under intensifying shipping, Suez Canal traffic, aquaculture, and climate warming. Yet, despite rapidly growing research activity, a comprehensive synthesis of the scientific literature on Mediterranean marine invasions has been lacking. This study provides the first Mediterranean-wide combined bibliometric and topic-modeling analysis of invasive marine species research, using 3521 unique documents retrieved from Scopus and Web of Science. We quantify temporal growth in publications and citations, map the conceptual structure of the field through co-citation, co-word, and topic modeling, and reveal pronounced regional and thematic biases. Latent Dirichlet Allocation resolves 13 coherent topics, dominated by first records of non-native species, invasive macroalgae, alien species diversity, and ecological impacts, with strong signals for Lessepsian migration and climate-driven range shifts, particularly in the Eastern Mediterranean. Spatial and thematic analyses reveal pronounced regional biases, with invasion hotspots in the Aegean and Levantine seas contrasted by comparatively sparse coverage of western and central sub-basins, and notable gaps in predictive modeling and socioeconomic assessments. The results underscore the need to rebalance effort toward under-studied regions and themes, while leveraging existing collaboration networks and methodological advances to support MSFD (Marine Strategy Framework Directive) implementation, International Maritime Organization (IMO) instruments, and broader ecosystem-based management. The reproducible framework presented here offers a baseline for periodically tracking research evolution and guiding adaptive, transboundary governance of Mediterranean marine bio-invasions.

Oceans doi: 10.3390/oceans7030036

Authors: Vasiliki Myrintzou Nikolaos Kokkos Dor Edelist Garabet Kazanjian Georgios Sylaios

Over the past two decades, Citizen Science (CS) has experienced rapid growth, driven by technological advancements and the rise of digital platforms. This work examines the necessity for standardization in Citizen Science data management and discusses how existing data standards can enhance the impact of citizen-generated data. CS standardization ensures data quality, comparability, reusability, and interoperability, making data suitable for contributing to the Marine Strategy Framework Directive (MSFD) and the United Nations Sustainable Development Goals (SDGs). This paper examined 130 Citizen Science publications and found that most collected data referred to the MSFD Descriptor 1 (Biodiversity—44.96%) and Descriptor 10 (Marine Litter—20.93%), followed by the alien species distribution (D2—11.63%), hydrography (D7—6.20%), eutrophication (D5—6.20%), and marine pollution (D8—3.10%). Analysis of 108 publications on SDG alignment revealed that the majority (35.58%) focused on reducing marine pollution. This paper reviews the best practices for effective Citizen Science data management, including standards for data structures, content, values, and exchange. Based on this review, Darwin Core, Ecological Metadata Language (EML), and the OGC SensorThings API appear to be the most suitable standards for MSFD-relevant CS data. Therefore, policymakers could enable the formal integration of standardized CS datasets into MSFD monitoring workflows.

Oceans doi: 10.3390/oceans7020035

Authors: Sara Ramos-Marin C. Guedes Soares

Harmonising heterogeneous met-ocean time series to a common temporal resolution is a prerequisite for integrated marine renewable energy assessments. Such datasets often differ in their sampling frequency, statistical distribution, and non-stationarity, complicating joint analysis. This study presents a practical multi-criteria framework for selecting temporal interpolation strategies for met-ocean datasets, explicitly balancing prediction accuracy and computational efficiency. Six environmental variables relevant to offshore renewable energy—wind speed, significant wave height, energy period, peak period, global horizontal irradiance, and upper-ocean thermal gradients—are analysed using ten-year reanalysis datasets for the Madeira Archipelago. Six commonly used deterministic time-domain interpolation methods are evaluated within a unified validation framework combining training–test splits, k-fold cross-validation, and Monte Carlo resampling. Their performances are quantified using the relative root mean square error and computational time, integrated through a composite performance score. The results show that makima interpolation provides the most consistent compromise between accuracy and efficiency for most variables in dense, regularly sampled met-ocean datasets, while spline-based approaches perform better for highly skewed solar irradiance. Preprocessing steps, such as detrending and distribution normalisation, yield only marginal improvements for dense, regularly sampled datasets, and method rankings remain stable under moderate changes in accuracy–speed weightings. Rather than proposing a universal interpolator, this work delivers a reproducible decision-support workflow for temporal resampling of multi-variable met-ocean datasets, supporting early-stage marine renewable energy assessments.

Oceans doi: 10.3390/oceans7020034

Authors: Zhibo Wang Qikun Li

To ensure safe and stable operation of towed array systems in complex marine environments, the concept of a Safe Maneuvering Envelope (SME) for towing maneuvers is proposed based on flexible cable dynamics theory. The dynamic equations of the towed array are established using the Lumped Mass Method. Using diving depth and breaking tension as boundaries, array configuration data sets are calculated for combinations of main cable outer diameter, vessel speed, and deployed cable length. Mapping relationships between vessel speed, cable deployment length, diving depth, and breaking strength are presented to construct the maneuvering safety envelope. This envelope defines the operational range where the array meets design maneuverability criteria. The safety envelope concept provides quantitative operational guidelines for towed array systems and offers crucial theoretical foundations and methodological support for safe system design and risk assessment.

Oceans doi: 10.3390/oceans7020033

Authors: Titus Karderic Williams Youssef Fannassi Zhour Ennouali Abdelahq Aangri Tarik Belrhaba Isaac Tukpah Aıcha Benmohammadi Ali Masria

This study presents a city-scale physical coastal vulnerability assessment of the 21 km Monrovia coastline (Liberia) using a multi-parameter coastal vulnerability index (CVI). Nine physical parameters—geology/geomorphology, shoreline change rate, elevation, slope, bathymetry, wave height, tidal range, relative sea level rise, and coastal landform characteristics—were integrated within an equal-weight ranking framework. The results identify spatially concentrated high vulnerability segments associated with low elevation, sandy geomorphology, and persistent shoreline retreat. The CVI represents a relative exposure screening rather than a predictive risk model. Limitations related to parameter weighting, classification dependency, and temporal heterogeneity are acknowledged. The findings support preliminary spatial prioritization for coastal adaptation planning

Oceans doi: 10.3390/oceans7020032

Authors: Nicola Ungaro Federica Lefons Annamaria Pastorelli Enrico Barbone

In recent decades, the issue of marine litter has emerged as a major environmental concern, particularly with regard to plastic litter. The European Marine Strategy Framework Directive (MSFD, 2008/56/EC) requires member states to monitor marine litter along the coast, in the water, and on the seabed. Since 2014, beach litter monitoring has been carried out in Italy’s coastal regions, an activity entrusted to the Regional Environmental Agencies System (ARPA). ARPA Puglia is responsible for monitoring the Apulian coastline, and this paper summarizes the main results obtained from 2014 to 2023. The monitoring, which was repeated twice a year, consists of a visual census of litter items along a 100-meter stretch of beach in six different locations across the Puglia region. During this period, an average of 506 litter items per 100 m were observed on the six target beaches in Puglia, 90% of which were plastic ones. Among these, single-use plastic items (SUPs) accounted for 37%. A trend analysis reveals a decline in the aggregate quantity of marine litter on Apulian beaches over the past decade, a phenomenon that is particularly evident when considering the SUP subcategory in isolation. This decreasing trend is consistent with the overall pattern observed along the Italian coastline and the coastlines of European seas. Consequently, it can be hypothesized that an increase in awareness of the issue, in conjunction with the implementation of European Directive 2019/904 for the reduction in single-use plastics, has resulted in more responsible practices. However, further efforts are needed to achieve the goal of 20 litter items per 100 m of beach to attain the Good Environmental Status under the Marine Strategy Framework Directive. The findings emphasize the importance of constant monitoring of litter items along the shoreline, as well as the integration of new and alternative methodologies (e.g., drone surveys) to evaluate the efficacy of European regulatory implementation.

Oceans doi: 10.3390/oceans7020031

Authors: Haotian Chen Hashmi Kaiser Jun Wang Xuexia Zhu

Quinolone antibiotic (QA) residues in various natural environments have recently received massive scientific attention. Nevertheless, there is limited information on the distribution characteristics and potential hazards of antibiotics in coastal wetlands. Here, the occurrence, spatial and seasonal distribution, and ecological risk assessment of eight QAs including pipemidic acid (PPA), ofloxacin (OFL), enrofloxacin (ENR), ciprofloxacin (CIP), sarafloxacin (SAL), lomefloxacin (LOM), flumequine (FLU), and oxolinic acid (OA) in coastal wetland were investigated through collected water, sediment, benthos, and plant samples along the Jiangsu coastline in four seasons. The results demonstrated that all selected QAs were detected with varying frequencies and degrees, and their mean concentrations in water, sediment, plants, and benthos ranged from n.d. to 6.11 ng L−1, 3.10 μg kg−1, 6.14 μg kg−1, and 17.13 μg kg−1, respectively. The seasonal differences in antibiotic concentration indicated higher values in winter and significantly lower values in summer, while no significant variations were observed between spring and autumn. Based on the risk quotient (RQ) method, the ecological risk assessment revealed medium risks for OFL, ENR, CIP, and LOM, and low or no risks of other QAs. It is suggested that the differences in PNEC values between seasons and toxicity of antibiotic mixtures should be considered in future studies for better illustration of actual risk levels. This research provides fundamental data and an assessment pattern that governments and other scientific groups all over the world could use as reference to evaluate QA residues in coastal wetlands.

Oceans doi: 10.3390/oceans7020030

Authors: Sarah Anne Abela Alan Deidun Adam Gauci Ritienne Gauci

Marine Protected Areas (MPAs) are essential for preserving marine biodiversity; yet they face challenges from various human pressures, including vessel activities. This study examines the extent, spatial distribution, and temporal variability of vessel activity within the Southwest Marine Protected Area (MT101), a Natura 2000 site off the Maltese Islands, with the aim of identifying where and to what degree different vessel categories overlap with protected marine habitats. Using Automatic Identification System (AIS) data spanning 2017–2022, a cumulative, normalised vessel density approach was applied to five vessel types: passenger, fishing, cargo, tanker, and tug and towing vessel, and spatially integrated with the distribution of four Annex I habitat types, including sandbanks, Posidonia oceanica meadows, reefs, and sea caves. The analysis reveals distinct spatial and temporal hotspots of vessel presence, with passenger and fishing vessels showing consistently high overlap with ecologically sensitive habitats, particularly within bay areas and along sections of the MPA boundary, while cargo, tanker, and tug activities are more concentrated offshore. While direct ecological impacts were not quantified and vessel density serves as a proxy for potential pressure, the results highlight areas where vessel-related pressures are likely to be most pronounced and where management intervention is most urgently required. By linking long-term vessel activity patterns with habitat distribution, this study delivers a spatially explicit and transferable framework for assessing cumulative maritime pressures, providing an evidence base to support targeted, habitat-specific management measures, improved enforcement, and marine spatial planning within MPAs.

Oceans doi: 10.3390/oceans7020029

Authors: Caixia Shao Dianjun Zhang Xuefeng Zhang

Ocean fronts play a vital role in modulating climate variability, driving material transport, and maintaining the stability of marine ecosystems. Therefore, accurate identification of ocean fronts is of great significance for marine environmental monitoring and resource management. This study focuses on the Northwestern Pacific region and conducts a systematic comparison between two representative deep learning models—U-Net and Mask R-CNN—for automated ocean front detection. The objective is to evaluate the adaptability and strengths of different network architectures in handling multi-scale features, complex background conditions, and boundary delineation, thereby providing a theoretical basis for model selection and application-specific deployment. Experimental results show that U-Net achieves superior spatial consistency in large-scale frontal segmentation, with an IoU of 0.81 and a Dice coefficient of 0.76, while maintaining relatively high computational efficiency. In contrast, Mask R-CNN demonstrates stronger boundary modeling capabilities in detecting small-scale fronts and handling heterogeneous backgrounds, achieving an IoU of 0.78 and a Dice score of 0.73, though at the cost of increased computational demand. Overall, U-Net is more suitable for broad-scale automatic detection of ocean fronts, whereas Mask R-CNN exhibits greater potential in complex scene recognition. Integrating the structural advantages of both models holds promise for further enhancing the stability and accuracy of frontal detection, thereby offering robust technical support for ocean remote sensing analysis and environmental forecasting.

Oceans doi: 10.3390/oceans7020028

Authors: Venceslav Delov Krastio Dimitrov Konstantin Mihaylov Georgi Terziyski Yana Stoyanova Petar Dimov Nikolay Nedyalkov Gergana Nikolova Boris Karakushev Nikolay Natchev

The Black Sea harbour porpoise (Phocoena phocoena relicta Abel, 1905) is an endemic cetacean with poorly understood spatial ecology in Bulgarian waters. This study aimed to update knowledge on its distribution, abundance, and habitat use throughout the Bulgarian Exclusive Economic Zone (EEZ). We conducted systematic aerial line-transect surveys in all four seasons between October 2022 and October 2023, combined with distance sampling and MaxEnt habitat modelling. Porpoises were present year-round across the EEZ, with marked seasonal shifts in distribution and habitat preferences. Highest densities were observed in spring, while winter distributions were concentrated offshore. Habitat suitability was dynamic, with key high-use areas identified near Cape Emine and in southern offshore waters near the Turkish border. Overall, about 40% of the EEZ represented high-suitability habitat. These findings provide the first comprehensive, year-round baseline for P. p. relicta in Bulgarian waters, highlighting the species’ flexible habitat use and seasonality. The study was conducted under extraordinary conditions due to regional military activity, which may have influenced porpoise behaviour and spatial patterns. The provided results are critical for designing effective conservation and management measures in the face of both natural and anthropogenic pressures and threats.

Oceans doi: 10.3390/oceans7020027

Authors: Patrizia Perzia Serena Zampardi Teresa Maggio Manuela Falautano Luca Castriota

Cassiopea andromeda is an invasive alien jellyfish that is increasingly reported across the Mediterranean Sea, yet its invasion dynamics and ecological implications remain poorly understood. This study provides an updated assessment of its spatial and temporal distribution, evaluates its potential impacts on ecosystem services and biodiversity, and explores management options through the 8Rs framework. An aggregated dataset of georeferenced records (1886–2025) was compiled from scientific literature and citizen-science platforms. Spatio–temporal analyses—including kernel density, key spatial distribution characteristics, spatial autocorrelation, and local hotspot detection—were applied to identify invasion phases, aggregation patterns, and directional dispersion. Results reveal two distinct invasion stages: a century-long arrival phase confined to the Levantine Basin, followed by an accelerated expansion since 2008, with a persistent hotspot in the eastern Mediterranean Sea and a westward dispersal trajectory. Evidence of ecological impacts within the Mediterranean Sea remains limited, however studies from other regions indicate both potential benefits and localized negative interactions with marine organisms. Application of the 8Rs model highlights implemented, feasible and challenging coordinated basin-wide strategies to support adaptive management of this alien resource.

Oceans doi: 10.3390/oceans7020026

Authors: Elisa Garcia-Gorriz Diego Macias-Moy Daniel González-Fernández Antonella Arcangeli Nuno Ferreira-Cordeiro Olaf Duteil Svetla Miladinova Ove Pärn Luis Francisco Ruiz-Orejón Eugenia Pasanisi Roberto Crosti Léa David

Accumulation of plastic litter in the marine environment is a pressing global concern. To study this issue, we use the Blue2 Modelling Framework (Blue2MF), an integrated modelling tool developed by the Joint Research Centre (JRC) of the European Commission. Our study uses the Lagrangian model LTRANS-Zlev (LTRANS) in the Blue2MF to simulate the trajectories, distribution, and accumulation of macroplastics in five European marine basins: the Baltic Sea, Black Sea, Mediterranean Sea, Atlantic Northwest European Shelf, and Atlantic Southwest European Shelf. By incorporating model-estimated macroplastic inputs from land and estimations of maritime (fishing) sources, we simulate distribution patterns of marine macroplastics between 2016 and 2018. Our study addresses the challenges involved in modelling the spatial distribution and abundances of macroplastics with the LTRANS model and the factors that may condition the estimation of the model accuracy when model results are compared/validated with marine litter observations available. We compare our model results with available observations, achieving a good agreement between predicted and observed macroplastic distributions and abundances and estimating the model accuracy for both beached and floating macroplastics. Our study provides a basis for future forecast runs to evaluate the impact of policy/management options on marine macroplastic pollution in European Seas.

Oceans doi: 10.3390/oceans7020025

Authors: Oussama Sabbar Bowen Zhang Jie Ge Longyan Wang

The wake that forms behind a yawing wind turbine is a complex flow region that can affect the performance of downstream turbines in offshore wind farms. It contains various flow features, including velocity deficit, shear layers, and vortex structures, which evolve in both time and space. Understanding this behavior is important for the design and operation of large-scale offshore wind farms. In this work, large-eddy simulations combined with proper orthogonal decomposition are used to study the wake development behind the National Renewable Energy Laboratory five-megawatt offshore wind turbine under both aligned and yawed inflow conditions. The results indicate that yawing the rotor leads to a lateral shift in the wake and increased asymmetry, with a stronger shear layer forming on one side. This asymmetry promotes enhanced mixing between the wake and the surrounding flow, contributing to a faster downstream recovery of the velocity field. The proper orthogonal decomposition analysis shows that the most energetic modes are associated with large-scale wake deflection and meandering, while higher-order modes correspond to smaller and less stable flow structures within the shear layer. The temporal evolution of these modes illustrates how the wake responds to the yaw maneuver and gradually reaches a new quasi-steady state. Overall, the study provides insight into the influence of yaw on wind turbine wake dynamics and demonstrates the applicability of combining large-eddy simulation with proper orthogonal decomposition for wake analysis in offshore wind farm studies.

Oceans doi: 10.3390/oceans7020024

Authors: Xiang Li Jiaqi Luo Yunfei Zhang Zhen Shi Jian Wang

The characteristics and causes of the long-term trends of diurnal variation of sea surface temperature (DSST) in the South China Sea (SCS) are investigated in this study based on the global hourly sea surface temperature data generated by the mixed layer model (MLSST) from the National Marine Environmental Forecasting Center (NMEFC) of China. Validation of the MLSST dataset demonstrates excellent agreement with in-situ buoy observations in the SCS with a correlation coefficient of 0.951, confirming its reliability in the SCS. Based on this dataset, the long-term trend of DSST in the SCS exhibits significant seasonal variations with the strongest magnitude in spring and the weakest in winter. Specifically, a significant decreasing trend of −0.0014 °C yr−1 during 1982–2009 transitioned to a pronounced increasing trend of 0.0057 °C yr−1 from 2010–2019. Both climatic factors and local atmospheric variables jointly modulate the DSST in the SCS. On the long-term timescale, the Pacific Decadal Oscillation (PDO) served as the dominant factor driving DSST changes in most areas of the SCS. After 2010, the PDO shifted to a persistent positive phase, providing a crucial climatic background for the basin-wide DSST increase. While the El Niño–Southern Oscillation (ENSO) showed enhanced correlation with DSST post-2010, the Indian Ocean Dipole (IOD) had negligible influence overall. In addition, the SCS summer monsoon played an important regulatory role in shaping the long-term trend of summer DSST by altering air–sea heat exchange processes. Among local atmospheric variables, sea surface wind speed was significantly negatively correlated with DSST, and net heat flux was significantly positively correlated with DSST, with their effects showing regional differentiation. The regulatory role of wind speed dominated in the western SCS, whereas the net heat flux exerted a more prominent impact in parts of the eastern SCS. This work clarifies the spatiotemporal patterns and multi-driver framework governing DSST variability in the SCS, providing a basis for understanding regional ocean–atmosphere interactions.

Oceans doi: 10.3390/oceans7020023

Authors: Ernesto Napolitano Adriana Carillo Roberto Iacono Gianluca Eusebi Borzelli Maria Vittoria Struglia

This study uses satellite altimeter data from the new AVISO dataset to investigate the coupling between sea level variability in the Sicily Channel and the Ionian Sea. The dataset spans the last three decades (1993–2024) and provides high spatial resolution coverage of the Mediterranean Sea (1/16°, or approximately 7 km). We analyze the variability of the sea surface height through Empirical Orthogonal Function and Singular Value Decomposition techniques applied to the Absolute Dynamic Topography. While the dominant modes of long-term variability reflect the known dynamics of the North Ionian Gyre, the singular value analysis allows us to identify a coherent spatial structure extending from the Sicily Channel to the Northern Ionian Sea. This provides the first observation-based, robust evidence of a dynamical coupling between the two basins, indicating that in the last thirty years the Northern Ionian Gyre is part of a broader, dynamically connected regional system integrating flows from the Sicily Channel. These findings are consistent with previous work, based on a hindcast simulation covering 1980–2010, in which we highlighted the key role of the Atlantic Ionian Stream in shaping interannual to decadal variability in the Northern Ionian Sea. Here, we extend the analysis to the present day, providing the most up-to-date, observation-based assessment of the regional dynamics.

Oceans doi: 10.3390/oceans7020022

Authors: Francesca Romana Reinero Marco Pireddu Giacomo Ridella Lorenzo Valenti Antonio Pacifico Francesca Ellero Primo Micarelli

The spinner shark is a widely distributed coastal species that faces significant anthropogenic pressures, yet information on its ecology in the western Indian Ocean remains poorly documented. This study provides preliminary baseline observations on temporal occurrence, sex ratio, and size distribution of a bait-attracted spinner shark aggregation in Hulhumale (North Malé Atoll, Maldives) and presents the first individual-level photo-identification (photo-ID) catalogue for the species based on underwater observations. Surveys were conducted in November 2024 and November 2025 using underwater photography, video recordings, and laser photogrammetry. In total, 69 individual spinner sharks were identified using the standard photo-ID protocol which proved to be valid. On the contrary, the preliminary application of the semi-automatic Identifin software indicated possible effectiveness for individual recognition; however, its performance cannot be reliably validated in this area because of poor image quality and environmental turbidity. Six individuals were re-sighted across years, demonstrating the feasibility of non-invasive repeated, long-term monitoring through photo-ID. Although interannual variation in sex ratio of sharks observed was detected (χ2 = 10.56, p = 0.0012), this pattern should be interpreted cautiously due to provisioning-related sampling bias and unequal sampling effort across years. Total length measurements (n = 28) indicated predominantly adult and subadult individuals, with no apparent interannual differences in size distributions. Overall, this study establishes a methodological baseline for spinner shark photo-ID in the Maldives and highlights the importance of multi-year and multi-season monitoring to robustly evaluate aggregation dynamics, site fidelity, and population-level patterns in this region.

Oceans doi: 10.3390/oceans7020021

Authors: Kirill Kivva Aleksandra Malysheva Aleksandra Sumkina

The Bering Sea and its surrounding waters are commercially and ecologically important ecosystems. Knowledge of phytoplankton phenology is crucial for understanding ecosystem dynamics. However, estimates of phenological parameters of spring phytoplankton bloom are sparse for this region. We used the Moderate Resolution Imaging Spectroradiometer (MODIS) daily data from 2003–2024 to assess the climatology of phenological parameters. A combination of data regriding, spatial interpolation, and temporal smoothing was applied. Three methods of spatial interpolation for missing data acquisition are compared: iterative first-order neighbor, inverse distance weighted interpolation, and data interpolating empirical orthogonal functions (DINEOF). We suggest that the first outcompetes the other two methods when compared to initial data. Date of the bloom initiation, bloom peak, chlorophyll-a maximum, and duration of the bloom before its peak are evaluated. The spatial distribution of mentioned phenological parameters is presented and discussed. We show that bloom starts early in Bristol Bay, in the narrow band along the eastern shelf, along the Kamchatka Peninsula, and south of the Aleutians and Alaska Peninsula. In the deep Bering Sea, bloom starts surprisingly later considering the latitude of the region. The main reason for this may be the wind mixing during the spring. The first phase of the bloom is generally longer in the deep southern areas (up to 60 days) and shorter in the northern shelf areas (less than 2 weeks in some cases).

Oceans doi: 10.3390/oceans7020020

Authors: Murray Thomson

Marine isopods are the ancestors of the harmless herbivorous woodlouse species often found in piles of leaves. In contrast to woodlice, marine isopods of the family Cirolanidae (cirolanid isopods) are scavengers and predatory carnivores that form swarms and can cause damage to fishing and aquaculture industries. Furthermore, these animals are known to bite swimming and diving humans and therefore may have negative effects on recreational activities in areas where swarms form. One cirolanid isopod species, Cirolana harfordi, displays social behaviour, an attribute that may facilitate the formation of swarms. This species gives live birth, a highly unusual mode of reproduction for an invertebrate and isopod. The rate of viviparous reproduction in C. harfordi is sped up by warmer conditions, indicating the threat that cirolanid isopods pose to ocean resources may intensify with global warming.

Oceans doi: 10.3390/oceans7010019

Authors: Patricia Silva Rodrigues Wilfried Bauer Marlon Carlos França

Understanding the impact of monsoonal oscillations during past climatic changes in the Arabian Sea is crucial for improving climate model predictions under ongoing global warming. This study investigates whether millennial-scale climate shifts in Greenland, specifically Dansgaard–Oeschger events 12–11, affected the Indian Ocean monsoon system and the associated productivity and oxygen minimum zone (OMZ) dynamics in the northwestern Arabian Sea. In the Arabian Sea, DO stadials correspond to reduced water-surface productivity, well-ventilated intermediate water masses, and a weakened or absent OMZ. Contrarily, DO interstadials are distinguished by enhanced water-surface productivity, a reorganization of intermediate water masses, and a reinvigoration of the OMZ. Eleven sediment samples from ODP Site 721A were analyzed using a multiproxy approach combining total organic carbon, C/N ratios, bulk-sediment isotopes (δ15N, δ13C), and the relative abundances of Globigerina bulloides and Globigerinoides ruber, complemented by isotopic data (δ13C, δ18O) from G. ruber shells. Further Mg/Ca–δ18O and δ18Osw measurements were included to refine the reconstruction of surface-water hydrography linked to productivity changes. Results reveal significant oscillations in water-surface productivity and OMZ intensity, modulated by shifts in monsoon strength and water-column ventilation. Enriched δ15N values, elevated TOC, and increased G. bulloides relative abundances reflect intensified denitrification and organic matter preservation under a stronger southwest monsoon, whereas depleted δ15N, reduced TOC, and higher G. ruber abundance indicate enhanced ventilation and a weaker OMZ under northeast monsoon dominance. These findings provide new evidence that refines the paleoceanographic history of the Arabian Sea. Additionally, they demonstrate that high-latitude climatic forcing during DO events modulated Arabian Sea monsoon dynamics and oxygenation through strong interhemispheric teleconnections.

Oceans doi: 10.3390/oceans7010018

Authors: Antonia Scicluna Patrick J. Schembri

The bearded fireworm Hermodice carunculata (Polychaeta) has become increasingly problematic in Mediterranean artisanal fisheries, yet remains understudied. This study provides a detailed analysis of interactions between H. carunculata and artisanal fishers in Marsaxlokk, Malta’s largest fishing village. Combining fisher interviews (local ecological knowledge) and field data, the study reveals that fishing practices inadvertently sustain and amplify local fireworm populations by discarding worms and by-catch nearshore, thereby providing abundant food sources. The highest fisher activity correlated with significantly denser fireworm populations and smaller-sized individuals, indicating population growth driven by fisher practices. Fireworm predation significantly impacts fishers economically, causing an estimated direct loss of 52.5% of the expected profit across the five onboard sessions, due to damaged catch, along with additional indirect losses from reduced gear efficiency and increased labor. The worm’s painful sting adds further nuisance and discomfort for fishers who frequently handle infested gear. Despite awareness of fireworm behavior, fishers largely did not recognize their practices as exacerbating the issue, creating a feedback loop perpetuating the problem. Behavioral experiments suggested that modifying fishing practices and gear design might mitigate fireworm impacts. Addressing this socio-ecological challenge requires further targeted research, education, and policy support to break the cycle that benefits fireworm proliferation in the region to the detriment of fishers.

Oceans doi: 10.3390/oceans7010017

Authors: Yaovi Zounon Zacharie Sohou Manuel Vargas-Yáñez Dogbè Clément Adjahouinou Legrand Sylvère Debleo Théophile Godome Francina Moya Ruiz M. Carmen García Martínez

The coastal zone of the Gulf of Guinea is characterised by the intensive harvesting of Crassostrea tulipa oysters, locally known as ‘Adakpin’ or ‘Atcha’, which serve as a vital source of income for local communities. This study aims to identify the optimal areas, depths, and seasonal periods that favour the growth of juvenile C. tulipa oysters in the coastal lagoon waters of southern Benin. Relatively uniform juvenile oysters were cultured at three depths (surface, mid-water, and bottom) in three production zones (Ahouandji, Dégouè, and Djondji) over the course of one year, covering the four climatic seasons of southern Benin. Juvenile oyster growth (in length, width, height, and weight) was monitored monthly. Simultaneously, key environmental variables (salinity, temperature, pH, water transparency, and dissolved oxygen) were measured in situ to evaluate their influence. A three-way ANOVA revealed that the month of the year had a significant main effect on oyster growth, defining two main growth periods: from October to December 2022 and from March to May 2023. Growth rates decreased during December 2022 to January 2023 and showed no growth from January to March 2023. Growth stopped again from May to September 2023, after the second growth period. Although the main effects of the farming level and production zone were not individually significant, significant two-way interactions were found for ‘Month × Depth’ and ‘Month × Production Zone’. This indicates that the effect of the month on growth depended on both the depth (farming level) and the location (production zone). Survival was highest at the mid-water column (97%) and at the surface (95%). Throughout the study period, survival rates remained consistently high, with the lowest monthly value at or above 80%.

Oceans doi: 10.3390/oceans7010016

Authors: Zhuo Fang Chen Shen Xun Han Cun Hu

Artificial reefs (ARs) are eco-friendly coastal protection infrastructures that mitigate wave-induced erosion while maintaining hydrodynamic connectivity and supporting ecological functions. This study evaluates the protective efficacy of a shellfish-algae reef system—a new type of AR—within the Houlong Bay coastal restoration project (Quanzhou, China) using an integrated numerical modeling approach. A coupled model system was established, incorporating MIKE 21 FM for hydrodynamics, MIKE 21 SW for waves, and MIKE ZERO ST for sediment transport, using unstructured triangular grids to resolve complex coastal topography. The model was validated against field data, including tidal currents and wave heights, showing good agreement. Pre-implementation simulations identified key coastal issues: insufficient wave attenuation in the southern fishery port segment, which results in localized erosion. Post-project simulations demonstrate that the novel integrated system—comprising shellfish-algae reefs, broad gentle beaches, and coastal vegetation—effectively reduced nearshore current speeds by approximately 0.15 m/s and attenuated significant wave heights by up to 70% during typhoon events. Short-term (1-year) sediment evolution showed mild deposition (0.1–0.8 m) at the toe of the artificial beach, which is consistent with design expectations. Long-term (10-year) simulations further confirmed coastal stability, with minimal long-term shoreline retreat (maximum 15 m) and low net alongshore sediment transport (annual average: 800 m3). This study provides a validated, data-driven reference for the design and implementation of AR-based restoration strategies in semi-enclosed bays, highlighting their dual role in erosion control and sustainable coastal management.

Oceans doi: 10.3390/oceans7010015

Authors: Hafssa Bensemlali Khalid El Khalidi Abdenaim Minoubi Nezha Mejjad Mohammed Bouchkara Hamid Nasrellah Bendahhou Zourarah

Oualidia lagoon, known as the oyster capital, is one of the most important coastal ecosystems along Morocco’s Atlantic coast. Over the past few decades, this ecosystem has faced numerous ecological challenges caused by both human activities and natural conditions, affecting its environmental quality. The present study aims to assess the impact of management projects implemented in the lagoon over the last eleven years on its health, especially in the sediment-trap area. In this context, a field mission was conducted in 2022, during which 15 surface samples were collected and analyzed using ICP-OES methods to determine metal concentrations. However, environmental indicators suggest that the ecological quality of the lagoon remains low, with significant and moderate contamination showing different trends, mainly for arsenic As (1–41.16 mg/kg); cobalt Co (2.01–7.2 mg/kg); molybdenum Mo (0–112.2 mg/kg); cadmium Cd (0.93–1.73 mg/kg); iron Fe (2433.36–19,721.55 mg/kg); and aluminum Al (640.7–11,600.57 mg/kg). The hotspots for these elements are mainly found at stations 13 and 15, which cover the upstream area of the lagoon near of the sediment trap. Comparing the results with those of previous studies conducted in the lagoon, there has been a decrease in sediment contamination since the sediment trap was created in 2011. The analysis suggests that different sources of these metals are entering the lagoon. This study provides updated data on metal concentrations in Oualidia lagoon sediments, one of the most diverse and biodiverse ecosystems in the Moroccan Atlantic. These results provide a scientific basis for targeted environmental management of the Oualidia lagoon, supporting priority monitoring and control of pollution sources. They also highlight the importance of developing awareness programs for residents, fishermen, farmers, food businesses, hotels, and guesthouse owners, alongside the continuation of management projects in the lagoon.

Oceans doi: 10.3390/oceans7010014

Authors: Weijia Song Jianguo Ding Liling Guan Jie Ma Huixuan Liu Hongli Zhang Yangjun Chen Kaiqin Jiang

Toxic dinoflagellate blooms and microplastics are widespread coastal pollutants. In this study, the scallop, Argopecten irradians, was selected as an experimental organism to systematically investigate the single and combined toxic effects of polystyrene (PS) and the toxic dinoflagellate, Alexandrium pacificum. The results showed that both PS and algal cells could be ingested by A. irradians. The survival rate of A. irradians remained above 90% in both the single and combined treatment groups, indicating that 1 mg/L PS and 1500 cells/mL A. pacificum cells did not pose a serious threat to scallop survival in the short term. However, CAT, SOD, and GSH-ST activities, as well as MDA content, were all elevated in the combined treatment group. Transcriptomic analysis further revealed that A. pacificum primarily affected immune-related pathways, whereas PS might interfere with endocrine function through the release of additives. Combined exposure to PS and A. pacificum induced more complex synergistic effects, reflected in the metabolic stress of exogenous substances, and the disruption of developmental and homeostasis regulatory pathways. This study provides important theoretical support for assessing the threats posed by composite coastal pollution to aquaculture and marine ecological security.

Oceans doi: 10.3390/oceans7010013

Authors: Mouhcine Medlouh Ibtissam Doukilo Ahmed Errhif Mohamed Id Halla Oum Keltoum Belhsen

The reproductive cycle of the European clam Ruditapes decussatus reared in suspended double-net trays in the Oualidia Lagoon was investigated from October 2017 to February 2019. This study aimed to characterize gonadal development through histological analysis, gonadal index assessment, and the biochemical composition of key macromolecules (proteins, lipids, and carbohydrates) over an annual cycle. The results revealed that R. decussatus undergoes a prolonged spawning period from April to December, with a peak in October when 100% of the population reached the maturation stage (stage IIIA). A sexual rest phase was observed between November 2017 and December 2017. An overall sex ratio of 1:0.8 was observed, indicating a slight female bias, with no significant deviations. Statistical analyses highlighted a correlation between the gonadal index and seawater temperature, suggesting that temperature plays a crucial role in regulating reproductive activity. Biochemical analyses showed that proteins were the predominant macromolecule in clam tissues, followed by lipids and carbohydrates. Seasonal variations in biochemical composition were observed; however, no direct correlation was found between biochemical compound levels and the gonadal index (p > 0.05). These findings provide valuable insights into the reproductive biology of R. decussatus under suspended aquaculture conditions, contributing to improved management and optimization of farming practices.

Oceans doi: 10.3390/oceans7010012

Authors: Alessandro Nota Thomas Hesselberg Francesco Tiralongo

The Atlantic bluefin tuna (Thunnus thynnus) is a high-value species subject to strict catch quotas and seasonal closures in the Mediterranean Sea. However, detecting illegal, unreported, and unregulated fishing remains challenging, particularly for small-scale activities. The aim of this study is to investigate whether citizen-reported strandings of tuna remains along Italian coasts can provide potential indirect indications of illegal fishing activities. To address this question, we collected and verified photographic records of stranded tuna remains (e.g., skulls, vertebral elements, complete specimens) reported on social networks. A similar search was conducted for two other conspicuous fishes, the ‘dusky grouper’ (Epinephelus marginatus) and the ‘greater amberjack’ (Seriola dumerili), as controls. Thirty-two strandings of tuna remains were recorded, with no records of amberjacks and only one of dusky grouper. Most strandings involved tuna heads or neurocrania, some of which showed clear mechanical cuts indicating post-capture processing. Several remains were older and degraded, suggesting long-term persistence in the sea. Overall, our results indicate that citizen-reported strandings could provide low-cost, spatially broad potential indications of discards resulting from illegal fishing activities. Wider adoption of standardised public reporting could not only engage the public in marine conservation, but also provide valuable data for policymakers tackling illegal fishing and complement traditional fisheries monitoring.

Oceans doi: 10.3390/oceans7010011

Authors: Bing Long Shuhan Chen Jingchun Zhou Dehuan Zhang Deming Zhang

Underwater imaging suffers from significant degradation due to scattering by suspended particles, selective absorption by the medium, and depth-dependent noise, leading to issues such as contrast reduction, color distortion, and blurring. Existing enhancement methods typically address only one aspect of these problems, relying on unrealistic assumptions of uniform noise, and fail to jointly handle the spatially heterogeneous noise and spectral channel attenuation. To address these challenges, we propose the variational-based spatial–spectral joint enhancement method (VSJE). This method is based on the physical principles of underwater optical imaging and constructs a depth-aware noise heterogeneity model to accurately capture the differences in noise intensity between near and far regions. Additionally, we propose a channel-sensitive adaptive regularization mechanism based on multidimensional statistics to accommodate the spectral attenuation characteristics of the red, green, and blue channels. A unified variational energy function is then formulated to integrate noise suppression, data fidelity, and color consistency constraints within a collaborative optimization framework, where the depth-aware noise model and channel-sensitive regularization serve as the core adaptive components of the variational formulation. This design enables the joint restoration of multidimensional degradation in underwater images by leveraging the variational framework’s capability to balance multiple enhancement objectives in a mathematically rigorous manner. Experimental results using the UIEBD-VAL dataset demonstrate that VSJE achieves a URanker score of 2.4651 and a UICM score of 9.0740, representing a 30.9% improvement over the state-of-the-art method GDCP in the URanker metric—a key indicator for evaluating the overall visual quality of underwater images. VSJE exhibits superior performance in metrics related to color uniformity (UICM), perceptual quality (CNNIQA, PAQ2PIQ), and overall visual ranking (URanker).

Oceans doi: 10.3390/oceans7010010

Authors: Esha Nauman Richard A. Lutz

The deep sea is often depicted as a barren environment. Using the abyssal plain as a baseline system characterized by high pressure, extreme nutrient limitation, and slow growth rates, this review contrasts these conditions with specialized habitats that serve as oases of life such as whale falls, cold seeps, and hydrothermal vents. These environments retain the high-pressure characteristic of deep-sea habitats, but other unique environmental factors select for organisms with distinct life-history strategies and growth rates. This review examines the environmental constraints, organism physiological adaptations, and life-history strategies that define each habitat. Through synthesizing these factors, we identify patterns that influence not only growth and succession, but broader ecosystem vulnerability and resilience, defined here as the capacity of these communities to recover from disturbance. By evaluating how biological traits contribute to resilience across the four habitats in response to specific environmental constraints, this comparative framework identifies trade-offs between growth specialization and habitat stability. Understanding these environmental factors is critical in evaluating the resilience of these habitats to growing anthropogenic disturbances and determining future directions of study. This review concludes that while hydrostatic pressure and temperature impose fundamental metabolic constraints, nutrient availability and habitat stability are the primary determinants of organismal growth rates and life-history strategies. In the context of each ecosystem, both these variables can play a large role in the ability and time to recover from disturbance and may be good indicators of resilience at both a community and an organismal level. Consequently, slow-growing, long-lived fauna may possess far lower intrinsic resilience to anthropogenic disturbance compared to rapidly growing organisms with shorter life histories. Varying resilience of these habitats may necessitate habitat-specific strategies for assessment and protection.

Oceans doi: 10.3390/oceans7010009

Authors: Davide Iaciofano Hadas Lubinevsky Sabrina Lo Brutto

Knowledge of deep-sea amphipods remains much more limited compared to that of shallow-water or more accessible marine habitats, although there has been an increasing scientific interest in recent decades. Deep-sea amphipods are mainly scavengers and detritivores, playing a role in organic matter recycling; however, their species richness may be underestimated, especially in understudied realms like the deep Mediterranean Sea. Long-term monitoring data are limited, hindering understanding of trends or human impacts. The present work aims to address this gap. In a previous study (1993–1996), twenty-two species of amphipods were identified from samples collected at depths between 734 and 1558 m along the Israeli coast. After twenty years, 16 sites were sampled in 2013 at depths ranging from 198 to 1812 m. Amphipod assemblage and its bathymetric distribution were analyzed to enhance knowledge of the taxon’s occurrence.

Oceans doi: 10.3390/oceans7010008

Authors: Geraldine Doolan Grace M. Cott Stephen Hynes

Under the requirements of the EU Nature Restoration Regulation, significant investment in the restoration of marine and coastal ecosystems will be necessary in EU Member States. A certain level of knowledge among the population will be required to ensure authorities have the required social license to pursue marine ecosystem restoration activities, and awareness raising campaigns will likely be a part of the body of actions taking place to support the delivery of the EU Nature Restoration Regulation. This survey assesses the baseline awareness, attitudes and support for marine and coastal restoration amongst the Irish adult population. Relatively low levels of awareness of both restoration initiatives and the regulation are documented, with approximately 15% of respondents aware of restoration activities and 19% aware of the regulation. While distance to the coast isn’t always strongly correlated with awareness and support, connection to the marine—represented by visiting the coast and being concerned about the marine environment—is strongly correlated with these metrics. The results indicate that information campaigns around marine and coastal restoration should attempt to highlight the benefits society receives from the marine and coastal environment and foster a sense of connection with it.

Oceans doi: 10.3390/oceans7010007

Authors: Rui Liu Liang Sun Haihua Liu Mengyuan Xu Gaopeng Lu Xiuting Wang Youfang Yan

Although previous studies composited response of sea surface temperature (SST) to typhoon sea surface wind (SSW) forcing around typhoon center, how SST responded spatiotemporally along the typhoon track over the ocean remains unclear. Through Empirical Orthogonal Function (EOF) analysis, several isolated typhoons in the Western North Pacific (WNP) from 2021 to 2024 were investigated. Two SSW forcing modes and two SST response modes were identified. The first SSW mode spatially reflects the overall distribution of SSW along the track, centering at its maturation position. And the first SST mode exhibits a high spatial correlation (|R|>0.85) with this SSW mode. The second SSW mode displays a distinct track-long scale dipole pattern along the path of the typhoon, representing its intensity variation during the “development–maturation–decay” lifecycle. Similarly, the second SST response mode shows a significant but lower correlation with this second SSW mode. Both corresponding SST response modes typically lag behind their respective wind-forcing by approximately 2 to 4 days, indicating that these SST response modes are direct reactions to SSW forcing. These cases implies that two track-long scale SSW modes are generally present during the lifecycle of typhoons and that their corresponding SST responses are dominated accordingly.

Oceans doi: 10.3390/oceans7010006

Authors: Federico Angel Velázquez-Muñoz Raul Candelario Cruz-Gómez Cesar Monzon

We use 29 years of altimeter-derived sea level anomalies and geostrophic velocities (1993–2021) from the Copernicus Marine Service to identify the Mexican Coastal Current (MCC) and to examine how it interacts with the coastline. Variance-ellipse and empirical orthogonal function analyses isolate a narrow alongshore jet with a mean width of about 95 km and average speeds near 0.3 m s−1 that reverses direction semiannually: poleward in June and July and equatorward in the rest of the year. When the MCC impinges on broad concavities in the coast, the boundary layer separates, forming recirculation cells that intensify and detach as coherent eddies. These near-shore eddies have similar radii (from ∼30 km) and relative vorticity of ±0.5×10−5s−1 at the beginning of their generation, and they propagate offshore once the current weakens. A simple numerical model reproduces the observed behavior and suggests that eddy formation is controlled by flow separation rather than generic instability. The semiannual change in direction of the MCC indicate a link with the larger-scale North Equatorial Countercurrent and Costa Rica Coastal Current systems of the eastern tropical Pacific.

Oceans doi: 10.3390/oceans7010005

Authors: Claire Angelina Guo Jiachi Zhao Eugene Pinsky

Changes in climate and ocean pollution has prioritized monitoring of ocean surface behavior. Ocean drifters, which are floating sensors that record position and velocity, help track ocean dynamics. However, environmental events such as oil spills can cause abnormal behavior, making anomaly detection critical. Unsupervised learning, combined with deep learning and advanced data handling, is used to detect unusual behavior more accurately on the NOAA Global Drifter Program dataset, focusing on regions of the West Coast and the Mexican Gulf, for time periods spanning 2010 and 2024. Using Density-Based Spatial Clustering of Applications with Noise (DBSCAN), pseudo-labels of anomalies are generated to train both a one-dimensional Convolutional Neural Network (CNN) and a Long Short-Term Memory (LSTM) network. The results of the two models are then compared with bootstrapping with block shuffling, as well as 10 trials with bar chart summaries. The results show nuance, with models outperforming the other in different contexts. Between the four spatiotemporal domains, a difference in the increasing rate of anomalies is found, showing the relevance of the suggested pipeline. Beyond detection, data reliability and efficiency are addressed: a RAID-inspired recovery method reconstructs missing data, while delta encoding and gzip compression cut storage and transmission costs. This framework enhances anomaly detection, ensures reliable recovery, and reduces energy consumption, thereby providing a sustainable system for timely environmental monitoring.

Oceans doi: 10.3390/oceans7010004

Authors: Sumit Kumar Ajit Kumar Akshatha Soratur Ankit Sarkar Balu Alagar Venmathi Maran

The open ocean cage aquaculture system is facing considerable challenges with disease outbreaks resulting from over-farming and the rise of resistance to antimicrobial treatment. However, the environmental consequences of antibiotic usage, including ecological contamination and the acceleration of antimicrobial resistance, underscore the urgent need for sustainable alternatives in aquaculture disease management. Lipopeptides, which are a compound that can be produced by marine bacteria such as Bacillus amyloliquefaciens or Bacillus subtilis, could represent a new solution. This review article comprehensively evaluates the feasibility of marine bacterial lipopeptides for sustainable disease management in open sea cage aquaculture. Lipopeptides, including surfactins, fengycins, iturins, and the clinically used daptomycin, have notable antiviral, antifungal, and antimicrobial properties, and can have positive effects on the immune system. Notably, lipopeptides have a remarkable antioxidant profile and excellent free radical scavenging ability, making them interesting candidates for improving disease resistance in fish relating to oxidative stress. The surfactins and iturins have amphiphilic structure and can destabilize pathogen cell membranes, inhibit biofilm formation and elicit host immune responses. This represents a paradigm shift in targeting multiple pathogens of aquaculture like Vibrio spp. and Aeromonas spp. Surfactins and iturins show broad-spectrum activity, while fengycins are selectively active against fungal threats. Daptomycin, which is primarily derived from Streptomyces, demonstrates the potential of the lipopeptide class to be developed therapeutically, which is something that tends to be overlooked. Unlike synthetic antibiotics, they are also biodegradable; therefore, there is much less environmental impact from lipopeptides. The complexity of the structure may have also some impact on the rate of development of resistance, if any. Their commercialization is possible; however, the main hurdles that need to be solved to improve aquaculture are the biologically scalable production, the economically viable purification, and the stability for practical application at sea. Integrating lipopeptides into disease management systems could also ensure the sustainability of open ocean cage aquaculture and reduce unnecessary antibiotic application.

Oceans doi: 10.3390/oceans7010003

Authors: André F. Jorge Carla Rodrigues Bernardo Quintella Marco Gomes da Silva Maria João Lança

Monitoring trace metals in commercially important fish species provides an early warning of anthropogenic contamination and potential risk to consumers. This study semi-quantified and quantified essential, non-essential, and toxic elements in the muscle of wild meagre (Argyrosomus regius) captured in the Tagus estuary (Portugal), which is used as a nursery and spawning aggregation area. Dry muscle was microwave-digested and analyzed using inductively coupled plasma–optical emission spectroscopy. Semi-quantified screening detected Al, B, Ca, Fe, K, Mg, Na, P, S, Si, Sr, and Ti, and eight elements were determined using multielement calibration (As, Cr, Cu, Hg, Mn, Ni, Se, and Zn); Cd, Pb (toxic elements), Co, and Mo were not detected in this study. Arsenic was detected in all individuals, with a minimum value of 0.348 mg/kg wet weight. A mercury level above the European Commission regulatory limit (0.5 mg/kg wet weight) was only detected in one individual, corresponding to 2% of the samples. Although other metals remain well below regulatory limits, continued biomonitoring is recommended to track temporal trends and safeguard seafood safety in transitional coastal systems, which is important for commercially relevant fish species.

Oceans doi: 10.3390/oceans7010002

Authors: Xi-Yu Xu Zhiyong Huang Tingting Shi Qiankun Liu Mengyao Li

In early 2025, the Jason-3 satellite’s orbit shifted from an “interleaved” to a tandem configuration with Sentinel-6A, and its Geophysical Data Records (GDR) were upgraded from Version F to G. This study evaluated GDR-G via eight processing approaches, using Jason-3’s last six GDR-F cycles (#394–#399) and first six GDR-G cycles (#501–#506), integrating histogram/geographical distribution analyses of Sea Surface Height Anomaly (SSHA), Significant Wave Height (SWH), Wind Speed (WS), and multi-method validation (e.g., self-cross-calibration). Key findings include the following: GDR-G had significantly lower SSHA noise than GDR-F, with up to ~4 cm SSHA bias from different retrackers/corrections; Adaptive retracker + 3D Sea State Bias (SSB) correction achieved optimal accuracy. Adaptive retracker’s SWH/WS anomalies linked to invalid MLE4 results and non-Brownian waveforms (coastal/sea ice). A detrending method was proposed, and the 41-point Lanczos window was optimal for smoothing. The results from the “detrending method” were consistent with the results based on the SSHA spectrum and classic self-cross-calibration methods. A ~5 mm drop was observed in Jason-3 GDR-G MLE4 baseline SSHA, probably caused by GDR upgrade or geographic sampling mismatch, while Sentinel-6A’s GDR-G upgrade might induce ~1 cm jump. The jumps along with GDR version upgrade highlighted the value of timely in situ absolute calibration.

Oceans doi: 10.3390/oceans7010001

Authors: Anderson B. Mayfield Alexandra C. Dempsey

Citizen science approaches for monitoring, and even restoring, coral reefs have grown in popularity though tend to be restricted to those who have taken courses that expose them to the relevant methodologies. Now that cheap (~10 USD), waterproof pouches for smart phones are widely available, there is the potential for mass acquisition of coral reef images by non-scientists. Furthermore, with the emergence of better machine-learning-based image classification approaches, high-quality data can be extracted from low-resolution images (provided that key benthic organisms, namely corals, other invertebrates, & algae, can be distinguished). To determine whether informally captured images could yield comparable ecological data to point-intercept + photo-quadrat surveys conducted by highly proficient research divers, we trained an artificial intelligence (AI), CoralNet, with images taken before and during a bleaching event in 2015 in Chagos (Indian Ocean). The overall percent coral covers of the formal, “gold standard” method and the informal, “tourist diver” approach of 38.7 and 35.1%, respectively, were within ~10% of one another; coral bleaching percentages of 30.5 and 31.8%, respectively, were statistically comparable. Although the AI was prone to classifying bleached corals as healthy in ~one-third of cases, the fact that these data could be collected by someone with no knowledge of coral reef ecology might justify this approach in areas where divers or snorkelers have access to waterproof cameras and are keen to document coral reef condition.

Oceans doi: 10.3390/oceans6040085

Authors: Karthika Padmini Ameen Ummath Atikulla Shaikh Balu Alagar Venmathi Maran

Holothuroids play a vital role in nutrient cycling and bioturbation to enhance the marine ecosystem. They enhance the biodiversity for various symbiotic marine organisms by providing essential shelter and spawning grounds. This review focuses on the Class Holothuroidea (Phylum Echinodermata) in Indian waters, encompassing a total of 187 species organized into 7 orders and 21 families. Notably, the order Holothuriida represents the largest proportion of species, accounting for 27%. These species are well-distributed across India, with the Andaman and Nicobar Islands exhibiting the highest level of species richness (107 species), followed by the East Coast (102 species), Lakshadweep (39 species), and the West Coast (34 species). Species diversity was assessed using the Shannon–Weiner diversity index. Results indicate that the Andaman and Nicobar Islands (H’ = 2.23) and the West Coast (H’ = 2.14) demonstrate the highest levels of diversity. This review provides a comprehensive and precise inventory of all species of Holothuroidea reported in Indian waters, which is provided to facilitate understanding of the reported species, their systematics, and distribution. In addition, a significant insight for both conservation and management of sea cucumbers in India has also been provided.

Oceans doi: 10.3390/oceans6040084

Authors: Kassandra Zannaki Ioannis E. Batjakas Gerasimos Kondylatos Maria Corsini-Foka Chrysoula Gubili Evangelia Krasakopoulou Ernesto Azzurro Emanuela Fanelli

Τhe trophic ecology of Pterois miles in the Mediterranean Sea was performed by integrating data from stomach contents (SCA) and stable isotopes analyses (SIA), based on samples caught off the Greek island of Rhodes, SE, Aegean Sea, for the first time. This combined approach provides information on ingested (SCA) and assimilated (SIA) food and thus allows for the depiction of predator–prey relationships. Specimens of devil firefish, including both juveniles and adults (total length of analyzed specimens spanned from 11.40 to 31.50 cm), were collected from different sites around Rhodes. Their diet consisted of bony fish, cephalopods, crustaceans, and gastropods. The δ13C and δ15N values ranged from −18.0 ‰ to −14.4 ‰ and from 7.2 ‰ to 9.2 ‰, respectively. SIA data allowed for the estimation of the trophic position of devil firefish from Rhodes Island, which showed a mean value of 3.1 ± 0.6 and confirms that the species primarily relies on a benthic baseline. Further, our isotopic values approach those obtained in North Carolina and Bermuda, confirming its role as a mesopredator in the Mediterranean benthic food webs. Although preliminary, such results can provide an important baseline for future investigations on the species and the potential impact on the Mediterranean food webs.

Oceans doi: 10.3390/oceans6040083

Authors: Mark R. Jury

Cape Town, South Africa, experiences coastal upwelling during austral summer. In this study, the effects of kinematic and thermal ‘roughness’ on wind stress are analyzed using 5–25 km resolution multi-satellite and coupled reanalysis datasets in the period 2010–2024. Average conditions for austral summer (December–February) are calculated to identify east–west gradients in sensible heat flux, wave height, and equatorward winds and to assess their consequences for the drag coefficient, wind-driven Ekman transport, and entrainment over the shelf from 16.9 to 18.7° E, north of Cape Town (33.7° S). Statistical and numerical outcomes are compared for austral summer and during active coastal upwelling in January 2018 with chlorophyll concentrations > 3 mg/m3. A subtropical anticyclone generated shallow equatorward winds next to a wind shadow north of Cape Town. Sharp cross-shore gradients in momentum flux were amplified by shoreward reductions in sensible heat flux and wave height, which suppressed the drag coefficient 10-fold. The inclusion of kinematic and thermal roughness in wind stress calculations results in a higher average cyclonic curl (−2.4 × 10−6 N/m3), which translates into vertical entrainment > 3 m/day at 33.7° S, 18° E. The research links coastal upwelling leeward of a mountainous cape with cross-shore gradients in air–sea fluxes that support recirculation and phytoplankton blooms during austral summer.

Oceans doi: 10.3390/oceans6040082

Authors: Xiaoyi Sun Yong Huang

Two new species of free-living marine nematodes were collected in the Yellow Sea, China, and they are described herein as Actinonema sinica sp. nov. and Comesoma zhangi sp. nov. Actinonema sinica sp. nov. is characterized by short cephalic setae; lateral differentiation consisting of a row of longitudinal sclerotized bars and beginning at the level of anterior third of the pharyngeal region; horn-shaped telamons; a curved rod-shaped gubernaculum; and an elongate conical tail with a smooth, pointed tip. Comesoma zhangi sp. nov. is characterized by long, thick cephalic setae, reaching up to 28 µm in length; a cup-shaped buccal cavity lacking a tooth and narrowing posteriorly with small projections; an amphidial fovea with two turns; slender, arcuate spicules 2.6 times the cloacal body diameter in length, lacking a proximal capitulum; a plate-like gubernaculum without apophysis; and the absence of precloacal supplements. Updated keys to the valid species of the genus Actinonema and the genus Comesoma are provided. A comparative morphological table of all currently accepted species of Comesoma is also provided.

Oceans doi: 10.3390/oceans6040081

Authors: Elena Calvo-Miguélez Belén Rodríguez-Fonseca Víctor Galván-Fraile Iñigo Gómara

The Mauritanian–Senegalese Coastal Upwelling exhibits strong interannual variability, which has been found to be driven by El Niño-Southern Oscillation (ENSO). In addition, ENSO has been shown to be triggered by the Indian Ocean and Atlantic Sea Surface Temperature (SST) variability. Nevertheless, how all these basins impact on the upwelling predictability has not been analyzed so far. Using a satellite product of surface chlorophyll-a as a proxy of marine productivity, this work makes an assessment of the predictability of the Mauritanian–Senegalese Coastal Upwelling marine ecosystem. Different statistical approaches are used to evaluate the relative contribution of the tropical basins, including the Pacific, Indian, equatorial and Tropical North Atlantic SSTs. The results indicate that although most of the upwelling variability stands for ENSO, the Atlantic contributions play an important role in shaping the seasonal prediction skill. These results may have strong implications for fisheries and marine ecosystem management in the region.

Oceans doi: 10.3390/oceans6040080

Authors: Gregory P. Asner Rachel R. Carlson Caleb Labo Dominica E. Harrison Roberta E. Martin

Coral life cycle dynamics are poorly understood in most reefs, especially at the large geographic scales commensurate with ocean transport, genetic flow, and other synoptic scale processes. We present a spatially explicit, large-scale, and multi-temporal study of coral settlement along a 30 km long reef system in the Southwest portion of Hawaiʻi Island. Here, we focused on interannual variability in coral recruitment from 2021 to 2024, a period without a major marine heatwave. We used stratified random site selection to place 320 coral settlement tiles at 32 sites (10 tiles per site) at 10 ± 3 m water depth annually to monitor recruitment of the three most common coral genera found in the region (Montipora, Pocillopora, Porites). Site-level interannual variability in coral recruitment was high yet the overall geographic distribution of recruits was consistent through time. This occurred despite a decrease in benthic temperature and recruitment rates during the study period. Persistent geographic patterns in coral recruitment strengthen our understanding of mechanisms and conditions that drive reef resilience. They also strongly suggest a need to protect areas of high recruitment while studying drivers of low recruitment in contrasting habitats. This approach will further increase support of coral production in an era of climate- and coastal pollution-driven declines in coral reefs.

Oceans doi: 10.3390/oceans6040079

Authors: Glacio S. Araújo Clarice S. Santigado Lucas C. B. Silva João Cotas Leonel Pereira

The present work aims to verify the growth (estimated with optical density) of the dry biomass after proper flocculation and weighing, and removal of phosphorus by the microalga Chaetoceros muelleri (Mediophyceae), at six different salinities. Cultivations were carried out with constant volume, for a period of eight days, consisting of six treatments with three repetitions each, at different salinities (30, 25, 20, 15, 10, and 5) (seawater = 34). We observed that the best results were obtained when the microalgae were grown at salinity 30, that is, we observed better performances for this microalga at higher salinities. At this same salinity, the microalgae presented the best results of phosphorus removal (46.08 ± 0.67%). Regarding biomass recovery by microalgae, after drying the flocculate, the best result was obtained at salinity 25, with a final value of 3.47 ± 0.04 g dry mass L−1. Therefore C. muelleri is a promising solution for increasing demand by the blue economy with the associated circular economy, promoting rehabilitation of ecological sites with economic output. Thus, this work aims to evaluate the effect of salinity on phosphorus removal using C. muelleri.

Oceans doi: 10.3390/oceans6040078

Authors: Eliza M. M. Mills Sarah Piwetz Dara N. Orbach

Bottlenose dolphins (Tursiops sp.) are opportunistic foragers with global distributions that utilize diverse feeding tactics based on environmental factors, habitat features, prey behavior, group dynamics, and genetics. We describe a unique foraging tactic regularly observed in the confluence of dredged shipping channels with high anthropogenic disturbance, and explore potential abiotic (temporal, tidal, habitat) drivers of the behavior. A shore-based digital theodolite was used from 2021 to 2022 to observe common bottlenose dolphins (T. truncatus) foraging within a current in a technique we term Orient-Against-Current (OAC). During OAC, dolphins position themselves facing into the flow of a current, swimming at a speed to maintain a stationary position within the current, and feed while prey move with the current towards them. Orient-Against-Current occurred in all seasons and throughout daylight hours, particularly during the winter and spring. Dolphins engaged in OAC during ebb tides and intermediate current speeds (1–2 knots), but not during slack tides. As OAC occurred closer to shoreline structures (i.e., seawalls, concrete blocks) than to mangroves and natural seagrass beds, it appears that hard human-engineered structures aid in prey capture during OAC. Knowledge of dolphin foraging techniques can aid in understanding behavioral plasticity shaped by anthropogenically altered environments in industrialized coastal areas.

Oceans doi: 10.3390/oceans6040077

Authors: Alban Souche Ebbe H. Hartz Lars H. Rüpke Daniel W. Schmid

Sediment plumes threatening benthic ecosystems are one of the environmental hazards associated with seafloor interventions such as bottom trawling, cabling, dredging, and marine mining operations. This study focuses on sediment plume release from hypothetical future deep-sea mining activities, emphasizing its interaction with turbulent ocean currents in regions characterized by complex seafloor topography. In such environments, turbulent lee waves may significantly enhance the scattering of released sediments, pointing to the clear need for appropriate impact assessment frameworks. Global-scale models are limited in their ability to resolve sufficiently high Reynolds numbers to accurately represent turbulence generated by seafloor topography. To overcome these limitations and effectively assess lee wave dynamics, models must incorporate the full physics of turbulence without simplifying the Navier–Stokes equations and must operate with significantly finer spatial discretization while maintaining a domain large enough to capture the full topographic signal. Considering a seamount in the Lofoten Basin of the Norwegian Sea as an example, we present a novel numerical analysis that explores the interplay between lee wave turbulence and sediment plume dispersion using a high-resolution Large Eddy Simulation (LES) framework. We show that the turbulence occurs within semi-horizontal channels that emerge beyond the topographic highs and extend into sheet-like tails close to the seafloor. In scenarios simulating sediment release from various sites on the seamount, our model predicts distinct behavior patterns for different particle sizes. Particles with larger settling velocities tend to deposit onto the seafloor within 50–200 m of release sites. Conversely, particles with lower settling velocities are more susceptible to turbulent transport, potentially traveling greater distances while experiencing faster dilution. Based on our scenarios, we estimate that the plume concentration may dilute below 1 ppm at about 2 km distance from the release site. Although our analysis shows that mixing with ambient seawater results in rapid dilution to low concentrations, it appears crucial to account for the effects of topographic lee wave turbulence in impact assessments related to man-made sediment plumes. Our high-resolution numerical simulations enable the identification of sediment particle size groups that are most likely affected by turbulence, providing valuable insights for developing targeted mitigation strategies.

Oceans doi: 10.3390/oceans6040076

Authors: Mitzi Paulina de La Rosa-Bernal Elizabeth Durán-Campos María Adela Monreal-Gómez Erik Coria-Monter David Alberto Salas-de-León Sergio Cházaro-Olvera

Chaetognaths play an essential role in zooplankton communities and significantly contribute to their overall biomass. Changes in the hydrographic properties of the water column, driven by hydrodynamic processes, affect their species richness and abundance. This study investigates the species richness and abundance of chaetognaths, as well as their relationship with circulation patterns at the boundary of the Pacific Ocean and the Gulf of California, Mexico. The analysis is based on high-resolution hydrographic data and zooplankton samples collected during the early summer of 2019. The results revealed a cyclonic circulation pattern that impacted the chaetognath community at depths greater than 200 m. This pattern resulted in higher chaetognath densities along the peninsular coast compared to the mainland coast. A total of 15 species from three different families were identified. Among these, Flaccisagitta enflata had the highest density, recorded at 16,143 ind 100 m−3, while Aidanosagitta neglecta exhibited a significantly lower density of only 48 ind 100 m−3. Multivariate statistical analyses indicated that hydrographic variables were key factors influencing the distribution of the chaetognath community during the sampling period. Given the significant research gap regarding this group in the region, our findings contribute to a deeper understanding of chaetognath communities and their relationship with circulation patterns in the Southern Gulf of California, recognized as an oasis of marine life.

Oceans doi: 10.3390/oceans6040075

Authors: Roberto Iacono Massimiliano Palma Ernesto Napolitano Alain Muñoz Caravaca

Due to the scarcity of in situ observations, the current description of the circulation around Cuba is far from complete. For example, the structure and variability of the flow through the Windward Passage, which hosts a significant fraction of the transport from the Atlantic to the Caribbean Sea, are still unclear. In this study, we use a recent, high-resolution Copernicus product based on satellite altimeter observations to obtain new insights into the large-scale geostrophic circulation around the eastern and southern coasts of Cuba. Among other results, we uncover a robust seasonal variability of the circulation around the Windward Passage, related to the presence of a cyclone to the south of the passage. Through most of the year the cyclone, with a companion anticyclone to the west, hinders the Atlantic inflow, but in autumn a strong stream crosses the western side of the passage and deeply penetrates the northern Caribbean Sea. The last part of the work deals with the time variability of the sea level in the Caribbean. We find that an apparent change in trend advocated in the recent literature has been reabsorbed in the last decade, yielding a local average sea level trend over the last thirty years in line with that for the global ocean.

Oceans doi: 10.3390/oceans6040074

Authors: Junmei Ou Shuangxin Wang Chuanhao Sun Wenyu Zhao Chenglong Jiang

Maritime accidents are low-probability, high-consequence events, making mechanism analysis crucial for risk mitigation. Existing studies often focus on single scenarios or factors and frequently mix pre-incident observational data with subjective unsafe behavior labels, limiting causal-chain construction for proactive risk prediction. To address these issues, this study proposes a Bow-Tie-based causal-chain Bayesian network, establishing a hierarchical inference chain of “observed parameters–unsafe causes–accident types” to capture causal interactions among multiple factor categories and enable inference from pre-incident data to potential unsafe causes and accident types. Applied to the Bohai Sea region, sensitivity analysis quantified the effects of risk factors under varying conditions on collision, sinking, and grounding probabilities. The results show that the method can infer accident types and unsafe causes using only pre-incident data, achieving over 70% accuracy and closely matching accident investigation findings. Moreover, it reveals layer-by-layer mechanisms of key contributing factors and provides targeted management interventions, supporting quantitative decision-making for maritime regulators and shipping companies, with significant practical applicability.

Oceans doi: 10.3390/oceans6040073

Authors: Qinsi Chen Yingyu Tan Song Hu Xiaohua Wang Heng Zhao Pengxia Liu Xing Liu

Tidal dynamics substantially govern nearshore circulation patterns. The discharge of sewage at different tidal stages may have a significant impact on the dilution of pollutants. However, discussions on tidal phase sewage discharge strategy are still rare. This study focuses on the narrow tidal passage in the Ningbo-Zhoushan sea area, which receives a large amount of coastal wastewater, but the role of the unique hydrodynamic processes in the dilution of pollutants in this region remains unclear. By using a combination of on-site measurements and the FVCOM-dye simulation method, the scenario of high-concentration sewage retention in the tidal passage was demonstrated. The coastal residual circulation formed by strong tidal currents confined over 78% of the tracers within a 3 km range near the shore, and a subsurface dye accumulation zone emerged along the 25–50 m isobaths. Monsoon transitions regulated pollution plumes, inducing 5–8% seasonal variability in pollution footprints controlled by wind-tide-stratification interplay. The tidal phase discharge strategy was revealed to be highly effective in this study; both submerged discharge in deep-water zones and intermittent discharge strategies implemented in shallow-water zones significantly reduce the spatial coverage of high-concentration sewage plumes. Our findings highlight the importance of formulating discharge strategies based on tidal phases in typical narrow and deep tidal passages.

Oceans doi: 10.3390/oceans6040072

Authors: Yijin Di Lingyu Zheng Jingzhao Ke Yinyin Zhou Shaoyang Mo Xiangbo Liu Jiquan Lin Yuxiao Ren Duanjie Huang Rouwen Chen Xiubao Li

Coral reefs, with their high biodiversity and ecological service functions, face significant threats due to climate change, overfishing, and pollution. The South China Sea (SCS) hosts rich coral biodiversity, particularly Acropora hyacinthus, a critical species for reef restoration. However, the region’s coral reefs are increasingly degraded, highlighting the urgent need for conservation efforts. In the present study, the genetic diversity and population structure of A. hyacinthus were examined based on two types of data: double-digest restriction-site-associated DNA (ddRAD) sequencing data and mitochondrial putative control region DNA (mtCR) sequences. Coral tissue samples were collected from 74 colonies inhabiting two inshore reefs (Sanya) and three offshore reefs (Xisha islands), and 748 single nucleotide polymorphisms (SNPs) and 74 mtCR sequences were obtained and utilized for downstream analysis. The results were consistent in analyses and did not cluster into two geographical groups for the inshore and offshore sites. Phylogenetic analysis showed that individuals of A. hyacinthus inhabiting the five detected sites were likely cryptic species HyaD. Furthermore, AMOVA and pairwise FST estimations based on both data types revealed no differentiation among five populations and between inshore and offshore reefs, which could be due to the reproductive mode of broadcast spawning for this species. However, a prevalent low level of genetic diversity was observed when compared with nearby Taiwan regions and Japan, and the geographic history results showed that the effective population size (Ne) had been decreasing for the past 300 years. Thus, we speculated that the populations of A. hyacinthus inhabiting the SCS lack the potential to cope with future climate change adequately, and multiple conservation measures should be implemented based on considering genetic diversity.

Oceans doi: 10.3390/oceans6040071

Authors: Ruth I. Ramírez-Villanueva Fernando Gumeta-Gómez Gustavo Hinojosa-Arango

Current scientific knowledge about the threats jeopardizing the recovery of sea turtle populations in Mexico is essential for improving conservation strategies. This study presents the results of a systematic review of the scientific literature published between 1960 and 2024, with the aim of identifying the main threats contributing to the decline of sea turtle populations in Mexico, detecting trends in scientific production, identifying knowledge gaps, and offering a comprehensive view of the challenges and priority actions needed to improve conservation strategies by region in Mexico. The review revealed a significant increase in research since 1997, with a predominant focus on pollution, fishing, and disease, mainly in the Pacific region. While Chelonia mydas and Lepidochelys olivacea were the most studied species, Dermochelys coriacea and Lepidochelys kempii received less attention. A total of 22 stressors associated with 8 threats were identified, but knowledge gaps persist regarding emerging threats such as climate change, coastal and marine development, and marine noise pollution. The results underscore the need to expand research on less-studied species and regions, strengthen studies in marine ecosystems, and improve the alignment between research and conservation policies to ensure the long-term viability of sea turtles in Mexico.

Oceans doi: 10.3390/oceans6040070

Authors: Johanna Kohler Mauvis Gore Rupert Ormond Katherine Mason Anne Veeder Timothy Austin

Sharks play a key role in coral reef ecosystems, but Caribbean populations are concerningly low. When monitoring endangered species, it is critical to use minimally invasive tools and protocols that are adequate for local species and the environment. This study investigated the adequate deployment time of baited remote underwater videos (BRUVs) for shark studies in the Cayman Islands and whether the use of photo-identification to recognise individuals (MaxIND) on BRUVs could improve abundance estimates (in comparison to MaxN) and the analysis of shark behaviour. From 2015 to 2018, a total of 557 BRUVs were deployed with recording times ranging from 3.8 to 211.03 min. The results showed that (1) of the total number of individual sharks recorded on videos, 95% of individuals were recorded within the first 110 min (slight variations between species), (2) MaxIND values were 1.1–1.5 times greater than that of MaxN (ratios varying with species) and (3) time of first arrival (Tarrive) was similar for all recorded species but time spent in front of the camera’s field of view (Tvisit) and activity levels (count of entries in camera’s field of view) varied between species. The results provide key information to improve the localised monitoring of rare/endangered species and can inform conservation management.

Oceans doi: 10.3390/oceans6040069

Authors: Gian Luca Eusebi Borzelli Ernesto Napolitano Roberto Iacono Maria Vittoria Struglia

The Ionian Sea plays a crucial role as a crossroads for various Mediterranean water masses, making it a significant factor in the seawater budgets, biogeochemistry, and biodiversity of the subbasins of the Mediterranean Sea. In recent years, numerous theories have been proposed in an effort to better understand the complex hydrography and dynamics of the Ionian. These theories primarily focus on the variability of the basin’s near-surface circulation, which is characterized by a recurring reversal that occurs over a period of 10–13 years. This variability is often attributed to internal processes and/or boundary forcing, as waters of Atlantic origin enter the basin from its western boundary. In this study, we utilize temperature–salinity profiles and absolute dynamic topography data provided by the Copernicus database to examine long-term changes in the vertical structure of the basin and their relationships with changes in the horizontal near-surface circulation. Our findings show that the vertical dependency of the density field of the basin undergoes significant fluctuations over interannual and decadal time scales, which induce important buoyancy changes throughout the water column and determine changes in the structure of the first baroclinic mode. However, no changes in the basin-averaged hydrographic structure can be related to the near-surface current reversals. These reversals are mainly associated with deformations of the main isopycnal surface, intended as the region of maximum buoyancy over the water column, suggesting that they do not impact the hydrographic properties of the various Ionian water masses. Instead, they alter their routes and relative volumes within different parts of the basin.

Oceans doi: 10.3390/oceans6040068

Authors: Jatdilok Titioatchasai Anuchit Darakrai Sinjai Phetcharat Jaruwan Mayakun

Increased seawater temperatures and nutrient loading are stressors that affect coral reefs and their microbiomes. In this study, filamentous algae were collected and exposed to different temperatures and nutrient concentrations through a laboratory experiment. Microbial DNA was extracted and analyzed using amplicon sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. In total, 1 domain, 51 phyla, 131 classes, 335 orders, 549 families, and 1905 species were identified. Proteobacteria and Bacteroidota were the dominant taxa reported. Elevated seawater temperatures and nutrient enrichment impacted microbial communities associated with turf algae under laboratory culture. Bacterial species diversity and abundance differed under different temperature and nutrient conditions. Proteobacteria and Actinobacteria were abundant in lower-temperature conditions, while Desulfobacterota, Spirochaetota, and Firmicutes were abundant in higher-temperature conditions. Ruegeria was abundant in low-temperature conditions, whereas Vibrio abundance was low. Regarding nutrient conditions, Proteobacteria and Cyanobacteria were abundant under high-nutrient conditions, while Firmicutes and Desulfobacterota were abundant under ambient-nutrient conditions. The higher nutrient concentration increased the abundance of pathogenic bacteria, such as Vibrio and Photobacterium, while Pseudoalteromonas, which is beneficial for reefs, was present under ambient nutrient conditions. This study demonstrates that temperature and nutrient enrichment can shape microbial communities under laboratory conditions, providing an experimental setting for further studies of bacterial functions and metabolic processes in natural conditions under thermal and nutrient stresses.

Oceans doi: 10.3390/oceans6040067

Authors: Laura Valentina Girón-Castaño Ann Carole Vallejo Isabel C. Avila Alan Giraldo

Whale-watching is a growing ecotourism activity in Colombia that offers economic benefits but may pose behavioral risks to humpback whales (Megaptera novaeangliae) if not properly managed. Although preventive recommendations exist to promote sustainable practices, their effectiveness remains largely unquantified in the Colombian North Pacific. This study evaluates adherence to whale-watching regulations and describes humpback whale behavioral responses to vessel presence in the Gulf of Tribugá, a key breeding and calving area. Data were collected from tourism vessels during July and September 2023, documenting 236 whales across 99 groups—71% of which included calves. The predominant whale response to vessels was neutral (74%), while evasive behaviors occurred in 22% of encounters, particularly among mother–calf pairs. Surface-active behavior was infrequent (22%) and most observed in calf–escort groups, serving as a supplementary indicator of behavioral state. Compliance with whale-watching recommendations varied: although most encounters involved a single vessel (57%) and averaged 16 min in duration, only 14% of vessels maintained the recommended minimum distance of 100 m. These findings highlight persistent gaps in regulatory adherence and underscore the need for strengthened enforcement and adaptive management to ensure sustainable whale-watching practices in Colombia’s North Pacific.

Oceans doi: 10.3390/oceans6040066

Authors: Amanda Kathryn Pappas Tahera Attarwala Gulnihal Ozbay

The increased presence of harmful algal blooms (HABs) is a concern for many aquatic environments, especially with the increasing effects of climate change. Members of the dinoflagellate genus Dinophysis have been shown to produce toxins that can cause Diarrheic Shellfish Poisoning (DSP) in humans who consume infected shellfish. The advancing oyster aquaculture industry in Delaware will require the development of management practices and monitoring HAB species to protect environmental and human health. Temperature, nutrients, and prey abundance can be drivers of Dinophysis blooms. D. acuminata has been historically identified at high concentrations (>200,000 cells L−1) in water samples from Rehoboth Bay, DE, USA. However, the reach of spring blooms and how far they extend to aquaculture sites have not been determined. This study monitored an emergent HABs threat of a toxin-producing dinoflagellate, Dinophysis acuminata, by assessing a transect during the first recorded winter bloom in Torquay Canal and analyzing concentrations of chemical nutrients of combined nitrate and nitrite, and orthophosphate. Pearson correlation coefficient analysis between cell density (cells L−1) and environmental variables across all sites was conducted to determine significant relationships between water temperature, Chl-a concentration, conductivity, dissolved oxygen (DO), combined nitrate and nitrite concentrations (NOx), and orthophosphate concentrations (PO43−). Genetic techniques and PCR were utilized to determine the presence of Dinophysis using genus-specific primers to monitor cell density or abundance within the sediments during winter months. There were no significant correlations between environmental variables, and nutrient concentrations did not exceed EPA regulations. Molecular analyses of benthic sediments detected Dinophysis spp., offering insight into potential bloom origins. Overall, there is limited ecological data on Dinophysis acuminata in Rehoboth Bay, DE, USA. The results of this study will help strengthen resources for monitoring HAB species and understanding potential risks to oyster aquaculture in Delaware.

Oceans doi: 10.3390/oceans6040065

Authors: Inês de Castro Susana C. Ribeiro António Louvado Newton Carlos Marcial Gomes Mário Cachão Paulo F. Silva Borges Eduardo Brito de Azevedo Joana Barcelos e Ramos

Ocean Alkalinity Enhancement (OAE) is a promising carbon dioxide removal strategy, but its ecological impacts on marine microbial communities under varying nutrient conditions remain poorly understood. We conducted laboratory incubations using a natural North Atlantic microbial assemblage to investigate the response to OAE under both natural and nutrient-enriched regimes. We tracked phytoplankton and bacterioplankton dynamics, biomass, and leucine aminopeptidase (LAP) and alkaline phosphatase (ALP) activity as indicators of organic matter remineralization. OAE consistently reduced phytoplankton abundance in both nutrient regimes, potentially due to CO2 limitation, resulting in lower production of phytoplankton-derived organic matter. This reduction was reflected in decreased LAP activity and shifts in the relative abundance of phytoplankton-associated bacterial taxa. These findings indicate that OAE can directly affect phytoplankton through carbonate chemistry alterations, with potential microbial responses largely mediated by changes in organic matter availability. While short-term microbial disruptions were modest, the ecological consequences of altered bloom dynamics should be carefully considered in future OAE deployment strategies.

Oceans doi: 10.3390/oceans6040064

Authors: Chairunnisa Br Sembiring Peng Zhang Jintian Xu Sheng Ke Jibiao Zhang

Macroplastic pollution is a growing environmental concern, threatening the marine environment. Despite growing awareness of marine plastic pollution, few studies have assessed the effectiveness of in situ technologies such as safety nets for macroplastic interception. This study aims to evaluate the effectiveness of safety net (SN) systems in intercepting macroplastic debris in the different zones of recreational Yugang Park Beach (YPB), Zhanjiang Bay, China. Safety nets were installed at stations representing different hydrodynamic conditions, and macroplastic debris (2.5–80 cm) was collected and analyzed for size, color, and shape characteristics. Two survey comparisons revealed a higher debris density in the winter survey (1.8 ± 0.3 items m2) than in the summer survey (1.5 ± 0.3 items m2). Most debris fell within the 10–40 cm range, with transparent low-density polyethylene plastic bags being the dominant type, particularly in the winter survey (80.7%). Statistical analysis indicated that plastic size was likely related to net retention characteristics, while tidal influences accounted for a major portion of spatial variability in debris accumulation. These findings suggest that SN systems are effective tools for macroplastic interception and could inform evidence-based coastal management strategies to reduce plastic pollution in similar coastal environments.

Oceans doi: 10.3390/oceans6040063

Authors: Molly Thornborrow Andrew S. Brierley Roland Proud Inigo Everson Joshua M. Lawrence Matteo Bernasconi Paul G. Fernandes

Antarctic krill (Euphausia superba) are an essential source of food for whale, seal, several fish, squid and seabird species in the Southern Ocean. Krill also play a major role in biogeochemical cycling and are the target of a growing commercial fishery. Krill can be detected and quantified with echosounders, particularly in swarms, and monitoring krill abundance and distribution is integral to assessing the status of regional populations and managing fisheries. We used echosounders to investigate the hemispherical distribution and behaviour of krill swarms during the Antarctic Circumnavigation Expedition (ACE), a multidisciplinary exercise that included measurements of atmospheric chemistry. Krill swarms were grouped using hierarchical clustering into four principal types: small swarms (on average 2 m high, 25 m long); large swarms (13 m high and 341 m long); deep swarms, which were also densely packed (average depth of 52 m); and shallower swarms, which had lower densities (average depth of 28 m). We found a weak negative relationship between the concentration of atmospheric methane close to the sea surface and the presence of krill. High densities of krill were found in the Amundsen Sea, an area purported to be of increasing importance for krill as the climate changes.

Oceans doi: 10.3390/oceans6040062

Authors: Ying Xu Wei Li Kuankuan Wu Sichong Ma Ma Guojun Wang Yuepeng Li Li Dayong Zhang

As a critical factor in ice load calculation for marine structures in cold regions, the growth mechanism and mechanical properties of rafted ice urgently require clarification. This study systematically investigated the growth patterns and flexural strength characteristics of rafted ice through laboratory-prepared specimens. Experimental results indicate that the thickness of rafted ice exhibits a negative correlation with both ambient temperature and initial ice thickness during growth. Due to the higher porosity of its frozen layer, the density of rafted ice decreases by approximately 8% on average compared to single-layer ice. Three-point bending tests demonstrate that, under the combined effect of high tensile strength in the lower ice layer and energy absorption by the porosity of the frozen layer, the flexural strength of rafted ice ranges from 1.12 to 1.34 times that of single-layer ice.

Oceans doi: 10.3390/oceans6040061

Authors: Fangru Li Xiaolei Liu Weiguo Hou Hailiang Dong Jinglong Hu Hongyu Chen Yi Ding Yuehong Wu Xuewei Xu

Hydrothermal vents, widely occurring along middle-ocean ridges and volcanic arcs, have been well-studied in vent-associated microbiology, mineralogy, and geochemistry. However, there are rarely investigations regarding the detailed microbial community in the hydrothermal vent-influenced sediment. To explore hydrothermal activities on microbial diversity at the Carlsberg Ridge in the northwestern Indian Ocean, four sediment cores were sampled from the near-vent fields to distant vent sedimentary fields in the Tianxiu hydrothermal field, and the microbial community compositions were analyzed. The sediment microorganisms closest to the hydrothermal vent were primarily composed of Acidimicrobiia, Gammaproteobacteria, Anaerolineae, and Planctomycetes. The microbial communities at the depth containing extensive signals of hydrothermal activity consisted mainly of Dehalococcoidia, Aerophoria, Anaerolineae, and Gammaproteobacteria. No significant differences in microbial composition were observed between the two weak hydrothermal sediment cores, primarily composed of Nitrososphaeria, Gammaproteobacteria, Alphaproteobacteria, and Acidimicrobiia. Moreover, heterogeneous selection substantially impacted the bacterial community assembly in near-vent sediments other than stochasticity. Multivariate statistical analysis identified that environmental fluctuations accounted for 55.59% of the community variation, with hydrothermal inputs (such as Fe, Pb, Cu, and Zn) being the primary factors shaping the construction of hydrothermal sediment microbial communities. These results enhance understanding of the response of deep-sea sediments to hydrothermal activity.

Oceans doi: 10.3390/oceans6030060

Authors: Deniece Melissa Aiken Ulla Pirita Tapaninen

Climate change has exacerbated the need for transitional shifts within high-impact sectors, notably maritime transport, which facilitates nearly 90% of global trade. In response, the International Maritime Organization (IMO) has implemented stricter environmental regulations under MARPOL Annex VI, which includes, among other things, the designation of Emission Control Areas (ECAs) and Particularly Sensitive Sea Areas (PSSAs). These regulatory instruments have prompted the uptake of new technologies, such as scrubbers, LNG propulsion, and low-sulfur fuels to mitigate emissions in these zones. However, emerging evidence has raised environmental concerns about these solutions which may offset their intended climate benefits. This study investigates the hypothesis that ECAs and PSSAs act as catalysts for maritime environmental advancements through a systematic mapping of 76 peer-reviewed articles. Drawing on data from Scopus and Web of Science, the study analyzes trends in technological advances, publication timelines, geographic research distribution, and the increasing role of decision-support tools for regulatory compliance. Findings show increased academic outputs particularly in China, North America, and Europe, and suggest that achieving effective emissions reduction requires globally harmonized policies, bridging research practice gaps, and targeted financial support to ensure sustainable outcomes throughout the sector. The study suggests that for ECAs and PSSAs to deliver truly sustainable outcomes, global regulation must be supported by empirical performance assessments, environmental safeguards for compliance technologies, and targeted support for developing maritime regions.

Oceans doi: 10.3390/oceans6030059

Authors: Dan Seidov Alexey Mishonov James Reagan

The North Atlantic Ocean is vital to Earth’s climate system. Scientific investigations have identified the Atlantic Meridional Overturning Circulation (AMOC) as a significant factor influencing global climate change. This circulation involves ocean currents that carry relatively warm, salty water northward in the upper layers, while transporting colder, less salty water southward in the deeper layers. The AMOC relies on descending water at deep convection sites in the high-latitude North Atlantic (NA), where warmer water cools, becomes denser, and sinks. A concern regarding the AMOC is that the freshening of the sea surface at these convection sites can slow it by inhibiting deep convection. Researchers have used oceanographic observations and models of Earth’s climate and ocean circulation to investigate decadal shifts in the AMOC and NA. We examined these findings to provide insights into these models, observational analyses, and palaeoceanographic reconstructions, aiming to deepen our understanding of AMOC variability and offer potential predictions for future climate change in the North Atlantic. While the influence of high-latitude freshwater is crucial and may slow the AMOC, evidence also shows a complex feedback mechanism. In this mechanism, the negative feedback from wind stress can stabilize the AMOC, partially counteracting the positive feedback effects of freshwater at high latitudes. Although some models predict significant shifts in AMOC dynamics, suggesting imminent and possibly severe deceleration, recent observational research presents a more cautious view. These data analysis studies acknowledge changes, but highlight the robustness of the AMOC, particularly in its upper arm within the Gulf Stream system. While it cannot be entirely dismissed that the AMOC may reach its tipping point within this century, an analysis of data concerning the decadal variability in the AMOC’s upper arm indicates that a collapse is unlikely within this timeframe, although significant weakening remains quite possible. Furthermore, deceleration of the AMOC’s upper arm could lead to less stable and more vulnerable North Atlantic Ocean climate patterns over extended periods.

Oceans doi: 10.3390/oceans6030058

Authors: Zhiyuan Hu Longqi Yang Zhenyu Sun Zhaozhang Chen Jia Zhu Jianyu Hu

This study examines the seasonal variability of surface currents in the South China Sea (SCS) and its adjacent regions, employing trajectory data from four seasonal deployments of Beidou drifters in the northern SCS. These observations are supplemented by reanalysis datasets, as well as satellite-derived sea surface wind and sea surface height data. The principal findings of this research are summarized as follows: (1) Drifter trajectories in the SCS exhibit pronounced seasonal characteristics. During autumn and winter, drifters predominantly move westward, ultimately merging with the SCS Western Boundary Current (SCSWBC). In spring, drifters are frequently entrained by mesoscale eddies. In summer, drifter trajectories generally move northeastward toward the Luzon Strait and the Taiwan Strait, with drifters subsequently returning to the SCS through these straits in autumn or winter before either joining the SCSWBC or settling in the coastal waters of Hainan. (2) The observed average drifter velocities show strong consistency with the CMEMS-reanalyzed current data during both the summer and winter seasons. (3) The surface current speeds along drifter trajectories in winter exhibit significant interannual variability, primarily driven by variations in wind speed. When the Niño 3.4 index exceeds ±0.5 °C (positive/negative phase), wind speeds and current speeds often reach their minimum (positive phase) or maximum (negative phase) values. These results enhance our understanding of the seasonal dynamics of surface currents in the SCS and their linkage to large-scale climatic variability.

Oceans doi: 10.3390/oceans6030057

Authors: Uwe Waller Klaus Kimmerle Harald Jensen

Atlantic salmon aquaculture has become an important seafood producer, contributing to the human diet. The natural productivity of Atlantic salmon populations is not sufficient to meet even a fraction of current aquaculture production, and it has not been able to do so in the past. Alternative process technologies are needed to maintain aquaculture production at current levels while mitigating the environmental impact along the coasts. Future aquaculture development must align with the UN Sustainable Development Goals. This study describes an aquaculture process vessel operating in the open sea and powered largely by renewable energy. Production conditions are fully adapted to the biology of salmon, improving production reliability, reducing coastal environmental impacts, and enabling more sustainable production. This study specifies the biological and technological aspects, provides evidence of the technical and economic feasibility, and justifies the relocation of salmon aquaculture to a large oceanic ecosystem, the North Atlantic Ocean.

Oceans doi: 10.3390/oceans6030056

Authors: Jisuk Ahn Changsin Kim Inseong Han Huitae Joo

To understand the impacts of climate change on local marine ecosystems, assessing ocean climate velocity in regional seas is essential. This study investigated changes in sea surface temperature (SST) and associated shifts in isotherm location and ocean climate velocity in the East Sea of Korea from 2000 to 2024, utilizing satellite-derived SST data. The results revealed a significant acceleration in the ocean climate velocity of SST, reaching 66.99 km/decade over the past 25 years. The velocity significantly increased during Phase 4, indicating rapid changes with potential ecosystem impacts. The 18 °C SST zone expanded by more than twofold from the early 2000s to the early 2020s. The annual average SST exhibited a steady, consistent decadal increase. These trends are associated with the northward shift of isotherms, which significantly influences the SST distribution patterns, particularly in the 16–18 °C range. Given the accelerating ocean climate velocity, urgent attention is needed to mitigate climate change impacts, particularly in vulnerable regions such as the East Sea. This study enhances the understanding of SST dynamics and underscores the importance of proactive conservation and management in climate-affected marine ecosystems.

Oceans doi: 10.3390/oceans6030055

Authors: Seyed Ahmadreza Hashemi Mastooreh Doustdar Abdullah Al Kindi Sachinandan Dutta

The population dynamics and exploitation ratios of the narrow-barred Spanish mackerel (Scomberomorus commerson, Lacépède, 1800) were investigated from August 2020 to February 2023, with data collected from three landing sites (Bushehr, Bandar Abbas, and Chabahar) of Iran waters. During the study period, only length was measured for 6504 specimens and both the length and weight were measured for 504 specimens. The mean fork length of the samples was 86 ± 20 cm, and the mean weight was 6230 ± 3742 g. The relationship between length and weight for the total samples was described by the equation W = 0.022 × CL2.76 (n = 504, R2 = 0.90, 95% C.I. for b = 2.52–2.91). The population dynamics indices for S. commerson were as follows: infinite length (Linf) = 173 cm, natural mortality (M) = 0.47 per year, growth coefficient (K) = 0.52 per year, total mortality (Z) = 1.42 ± 0.06 (95% C.I. = 1.36–1.48), fishing mortality (F) = 0.95 per year, and exploitation coefficient (E) = 0.67. The exploitation rate (U) and total stock at the beginning of the year (B0) were 0.6 and 48,333 tons, respectively. The annual average standing stock (Bt) was estimated at 30,526 tons. The exploitation ratio for maximum sustainable yield (EMSY) was 0.50, and fishing mortality at maximum sustainable yield (FMSY) was 1.5. The estimated range for maximum sustainable yield (MSY, in 1000 tons), the B/BMSY ratio, F/FMSY ratio, and saturation (S) ratio of S. commerson in the Iranian part of the Persian Gulf and the Sea of Oman was 20 (17–25), 1.55 (1.25–1.73), 0.90 (0.8–1.12), and 0.45, respectively. The stock of S. commerson is approaching overfishing in Iran waters, imposing immediate management actions to reduce catch and fishing effort.

Oceans doi: 10.3390/oceans6030054

Authors: Jian Wei Joanna J. Waniek Joachim Kuss Oscar Beltran-Perez Jen-Ping Peng Chengchun Shi Jin Chen Jihui Liu Lili Chen Rongrong Xie

The oxygen depletion in worldwide oceans and inland waters is becoming an increasingly prevalent problem. Here, two comparative study sites, Baltic Proper (BP) and Shuikou Reservoir (SR), were selected to identify the dynamics of dissolved oxygen (DO) and the related physicochemical response by conducting five field investigations over one year period. The DO concentrations were 0–9.9 mg L−1 and 0.3–8.7 mg L−1 in BP and SR, with average oxygen change rates of −0.016~0.014 mg L−1 d−1 and −0.022~0.018 mg L−1 d−1, respectively. Such oxygen dynamics were highly related to salinity, temperature, turbidity, and chlorophyll-a than nitrogen and phosphorus. The persistent hypoxia (i.e., DO < 2 mg L−1) emerged below 63 m in BP during all sampling months where the reversal points of Brunt–Väisälä frequency N expressed in the form log10[N2 (s−2)] were −4. The seasonal hypoxia covered the downstream of SR at depths below 11.5 m in September with the highest log10[N2 (s−2)] between −3.95 and −3.64. The log10[N2 (s−2)] ≈ −4 may indicate the occurrence and development of hypoxia in both regions. In the case of the persistent and seasonal hypoxic conditions in BP and SR, the impact of temperature and turbidity on DO was opposite. Low oxygen levels in SR occurred under elevated temperature and turbidity. Additionally, under hypoxic conditions in both regions, NO3− and NH4+ concentrations significantly depended on DO changes. This study contributes to the understanding of seasonal changes in oxygen dynamics in different water bodies, and highlights different oxygen conditions and physicochemical responses to the oxygen changes.

Oceans doi: 10.3390/oceans6030053

Authors: Sivakrishnan K. Kalappurakal Shanas R. Puthuveetil V. Sanil Kumar

Multimodal waves can significantly impact ocean–atmosphere interactions and affect coastal ecosystems. Due to the presence of waves created in different geographical areas, many wave systems coexist in coastal seas. Based on data collected with a directional waverider buoy, this study investigates fluctuations in multimodal sea states from March 2010 to May 2020 in the eastern Arabian Sea. The watershed-based spectral partitioning method is used to analyze 2D wave spectra obtained from measurements. Four-wave systems are present during pre- and post-monsoon periods, and three systems are detected during the monsoon (June–September). Interannual changes in significant wave height and peak wave period of different systems are investigated, revealing the maximum interannual variability of all wave systems in the inter-monsoon periods (May and October). The most energetic system during the pre-monsoon period is wind seas from the northwest direction, whereas during monsoon, swells from the southwest-west dominate. This pattern is similar across a spatial distance of 570 km along the western coastal waters of India. In the post-monsoon period, both systems (wind seas and swells) are present, with swells having slightly higher intensity.

Oceans doi: 10.3390/oceans6030052

Authors: Hafez Ahmad Felix Jose Padmanava Dash Shakila Islam Jhara

Mesoscale eddies have a significant influence on primary productivity and upper-ocean variability, particularly in stratified and monsoon-driven basins like the Bay of Bengal (BoB). This study analyzes mesoscale eddies in the BoB from January 2010 to March 2020 using post-processed and gridded daily sea surface height anomaly (SLA) data from the Copernicus Marine Environment Monitoring Service. We used a hybrid detection method combining the Okubo–Weiss parameter and SLA contour analysis to identify 1880 anticyclonic and 1972 cyclonic eddies. Cyclonic eddies were mainly found in the western BoB along the east Indian coast, while anticyclonic eddies were less frequent in this area. Analysis of eddy lifespans revealed that short-lived (1-week) eddies were nearly equally distributed between anticyclonic (48.81%) and cyclonic (51.19%) types. However, for longer-lived eddies, cyclonic eddies became more prevalent, comprising 83.33% of 30-week eddies. A notable, consistent eddy presence was observed east of Sri Lanka, influencing the East India Coastal Current. Most eddies (91%) propagated west/southwestward along the western slope of the Andaman Archipelago, likely influenced by ocean currents and coastal topography, with concentrations in the Andaman Sea and central BoB. These patterns suggest significant interactions between eddies, coastal upwelling zones, and boundary currents, impacting nutrient transport and marine ecosystem productivity. This study contributes valuable insights into the dynamics of ocean circulation and the impacts of eddies, which can inform fisheries management strategies, advance climate resilience measures, expand scientific knowledge, and guide policies related to conservation and sustainable resource utilization.

Oceans doi: 10.3390/oceans6030051

Authors: Alizé Deguette Katia Pes Bernard Vasconcellos Monya Costa João Silva Isabel Barrote

Glyphosate-based herbicides (GBHs) are extensively used worldwide, raising concerns about their potential effect on non-target aquatic ecosystems. This study investigated the short-term physiological effects of a commercially available GBH on the seagrass Zostera marina under controlled mesocosm conditions. Z. marina individuals were exposed to three concentrations of glyphosate (0.165, 51, and 5100 mg L−1) for 4 days, and the impacts on photosynthetic performance, growth rate, photosynthetic pigments content and energy metabolism were assessed. Exposure to 5100 mg L−1 of glyphosate caused rapid water acidification and complete plant mortality within 24 h. Exposure to 51 mg L−1 of glyphosate significantly impaired photosynthetic efficiency and foliar growth rate. Energy availability, photosynthesis and photosynthetic pigments content were highly disrupted at both higher concentrations. Exposure to 0.165 mg L−1 of glyphosate decreased the foliar chlorophyll a/b ratio. These findings show that Z. marina can potentially be threatened by the presence of GBHs even at lower concentrations and underscore the necessity for monitoring herbicide pollution in coastal waters to protect seagrass habitats and associated ecosystems. Further research is needed to assess long-term effects and the role of herbicide formulations in mediating toxicity.

Oceans doi: 10.3390/oceans6030050

Authors: Lucía Rivas-Iglesias Eva Garcia-Vazquez Verónica Soto-López Eduardo Dopico

Maritime traffic accounts for more than 90% of world trade. Noise, pollution, and litter are its drawbacks, affecting especially vulnerable migratory fish. Here, a motivated team of citizen scientists analyzed maritime traffic from three estuaries of the south Bay of Biscay and three from the south of the Iberian Peninsula, where the European eel is critically endangered, during the season of the entrance of glass eels. More than 164,000 data points about ship types and positions were collected. The results showed that traffic differences between estuaries would explain, at least partially, the different eel conservation statuses. The participants appreciated learning about ships and nature conservation and acquiring an awareness of the real volume of shipping and its potential impacts. All the citizen scientists, new and experienced, would like to get involved in ocean research again.

Oceans doi: 10.3390/oceans6030049

Authors: Haochen Zhang Atsushi Ooki Tetsuya Takatsu Atsushi Yamaguchi

Funka Bay, located in southwest Hokkaido, is a vital fishing area with a shallow depth of less than 100 m. Seasonal flows of the Oyashio and Tsugaru Warm Current affect the marine environment, leading to significant changes in zooplankton communities, yet limited information is available on these variations. This study used ZooScan imaging to analyze seasonal and interannual changes in zooplankton abundance, biovolume, community structure, and size composition from 2019 to 2023. Water temperature was low in March–April and high in September–November, with chlorophyll a peaks occurring from February to April. Notable taxa such as Thaliacea, Noctiluca, and cladocerans were more common in the latter half of the year. Interannual variations included a decline in large cold-water copepods, Eucalanus bungii and Neocalanus spp., which were abundant in 2019 but decreased by 2023. Zooplankton abundance and biovolume showed synchronized seasonal changes, correlating with shifts in the Normalized Biovolume Size Spectra (NBSS) index, which measures size composition. Cluster analysis identified eight zooplankton communities, with Community A dominant from July to December across all years, while Community D was prevalent in early 2019 but was replaced in subsequent years. Community E emerged from March to April in 2021–2023. In 2019, large cold-water copepods were dominant, but from 2020 to 2023, appendicularians became the dominant group during the March–April period. The decline in large copepods is likely linked to marine heat waves, influencing yearly zooplankton community changes.

Oceans doi: 10.3390/oceans6030048

Authors: Zhihao Tian Bing He Heng Zhang Cunzhe Zhang Tongrui Zhang Runfeng Zhang

The Kuroshio Current’s flow velocity imposes exacting requirements on underwater vehicle propulsive systems. Ecological preservation necessitates low-noise propeller designs to mitigate operational disturbances. As technological evolution advances toward greater intelligence and system integration, intelligent unmanned systems are positioning themselves as a critical frontier in marine innovation. In recent years, the global research community has increased its efforts towards the development of high-maneuverability underwater vehicles. However, propeller design optimization ignores the key balance between acoustic performance and hydrodynamic efficiency, as well as the appropriate speed threshold for blade rotation. In order to solve this problem, the propeller design of the NACA 65A010 airfoil is optimized by using OpenProp v3.3.4 and XFlow 2022 software, aiming at innovating the propulsion system of shallow water agile submersibles. The study presents an integrated design framework combining lattice Boltzmann method (LBM) simulations synergized with fully Lagrangian-LES modeling, implementing rotational speed thresholds to detect cavitation inception, followed by advanced acoustic propagation analysis. Through rigorous comparative assessment of hydrodynamic metrics, we establish an optimization protocol for propeller selection tailored to littoral zone operational demands. Studies have shown that increasing the number of propeller blades can reduce the single-blade load and delay cavitation, but too many blades will aggravate the complexity of the flow field, resulting in reduced efficiency and noise rebound. It is concluded that the propeller with five blades, a diameter of 234 mm, and a speed of 500 RPM exhibits the best performance. Under these conditions, the water efficiency is 69.01%, and the noise is the lowest, which basically realizes the balance between hydrodynamic efficiency and acoustic performance. This paradigm-shifting research carries substantial implications for next-generation marine vehicles, particularly in optimizing operational stealth and energy efficiency through intelligent propulsion architecture.

Oceans doi: 10.3390/oceans6030047

Authors: Ángel Urzúa Marina Gebert

Global marine coastal aquaculture increased by 6.7 million tons in 2024, with whiteleg shrimp (Penaeus vannamei) dominating crustacean production. However, reliance on a single species raises sustainability concerns, particularly in the face of climate change. Diversifying shrimp farming by cultivating native species, such as the European brown shrimp (Crangon crangon), presents an opportunity to develop a sustainable blue bioeconomy in Europe. C. crangon holds significant commercial value, yet overexploitation has led to population declines. Integrated Multi-Trophic Aquaculture (IMTA) offers a viable solution by utilizing fish farm wastewater as a nutrient source, reducing both costs and environmental impact. Research efforts in Germany and other European nations are exploring IMTA’s potential by co-culturing shrimp with species like sea bream, sea bass, and salmon. The physiological adaptability and omnivorous diet of C. crangon further support its viability in aquaculture. However, critical knowledge gaps remain regarding its lipid metabolism, early ontogeny, and reproductive biology—factors essential for optimizing captive breeding. Future interdisciplinary research should refine larval culture techniques and develop sustainable co-culture models. Expanding C. crangon aquaculture aligns with the UN’s Sustainable Development Goals by enhancing food security, ecosystem resilience, and economic stability while reducing Europe’s reliance on seafood imports.