Oceans, Volume 6, Issue 3 (September 2025) – 22 articles

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. Full article

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. Full article

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. Full article

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. Full article

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 × CL^2.76 (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 (B_t) was estimated at 30,526 tons. The exploitation ratio for maximum sustainable yield (E_MSY) was 0.50, and fishing mortality at maximum sustainable yield (F_MSY) was 1.5. The estimated range for maximum sustainable yield (MSY, in 1000 tons), the B/B_MSY ratio, F/F_MSY 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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

This study focused on the carbonate system dynamics and air–sea CO₂ fluxes in the open-ocean waters of the Madeira–Tore Seamount Complex during August 2024. Surface water properties revealed pronounced latitudinal gradients in sea surface temperature (21.9–23.1 °C), salinity (36.2–36.7), and dissolved oxygen (228–251 µmol Kg⁻¹), influenced by mesoscale eddies and topographically driven upwelling. Despite oligotrophic conditions, distinct phytoplankton assemblages were observed, with coccolithophores dominating southern seamounts and open-ocean stations, and green algae and diatoms indicating episodic nutrient input. Surface total alkalinity (TA: 2236–2467 µmol Kg⁻¹), dissolved inorganic carbon (DIC: 2006–2183 µmol Kg⁻¹), and pCO₂ (467–515 µatm) showed spatial variability aligned with water mass characteristics and biological activity. All stations exhibited positive air–sea CO₂ fluxes (2.8–11.5 mmol m⁻² d⁻¹), indicating the region is a CO₂ source during summer. Calcite and aragonite saturation states were highest in stratified, warmer waters. Principal Component Analysis highlighted the role of physical mixing, carbonate chemistry, and biological uptake in structuring regional variability. Our findings emphasize and contribute to the complex interplay of physical and biogeochemical drivers in modulating carbon cycling and ecosystem structure across Atlantic seamounts. Full article

This study proposed a sea cliff stability model that accounted for the coupled effects of sea erosion and rainfall, offering an improved quantitative assessment of the toppling risk. The approach integrated the notch morphology (height and depth) and rainfall infiltration to quantify stability, validated by field data from six toppling sites near Da’ao Bay, where the maximum erosion distance error between model predictions and measurements ranged from 0.81% to 48.8% (with <20% error for Sites S2, S3, and S4). The results indicated that the notch morphology and rainfall exerted significant impacts on the sea cliff stability. Site S4 (the highest site) corresponded to a 17.5% decrease in K per 0.1 m notch depth increment. The rainfall infiltration reduced the maximum stable notch depth, decreasing by 8.86–21.92% during prolonged rainfall. This model can predict sea cliff stability and calculate the critical notch depth (e.g., 0.56–1.22 m for the study sites), providing a quantitative framework for coastal engineering applications and disaster mitigation strategies under climate change scenarios. Full article

The population of southern fin whales (Balaenoptera physalus quoyi) was severely depleted by 19th and 20th century whaling. Its conservation status remains ‘vulnerable’, as recovery has been slow. Over 19 years of underwater acoustic recordings from the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO)’s hydrophones off Cape Leeuwin, Western Australia, were analyzed to monitor fin whales’ annual migration from their Southern Ocean feeding grounds (where they spend the austral summer) to their tropical breeding grounds (where they spend the austral winter) and back. Northward migrants arrived ~2 d/y earlier (2002–2020). The number of hours with fin whale acoustic presence increased by ~49 h/y and the number of days with fin whale acoustic presence by ~3 d/y. Thus, by the end of the 19-year recording period, fin whales were acoustically present on 74 more days than at the beginning of recording. While changes in habitat function, climate, and ambient noise may affect migratory behavior, the most likely explanation is a post-whaling increase in the number of animals of this Southern Hemisphere subspecies. Full article

A small-sized shark was accidentally caught in the coastal waters of southeastern France and abandoned in the port of L’Estaque (Marseille) in May 2024. An autopsy revealed that it was a young female shortfin mako, Isurus oxyrinchus (Rafinesque, 1810). Numerous superficial wounds on the head, back, and flanks, in the form of parallel striations resulting from heavy friction, indicate meshing in a gillnet that provoked the animal’s death by asphyxiation. Its very small size (64.77 cm TL) suggests that this is a newborn individual, leading to the hypothesis that the waters off Marseille could be a nursery area for this species. This observation alone is not enough to prove this point, but it does suggest the need for closer monitoring to validate it. Should the area be proven to play this key role for this critically endangered species, measures to limit fishing pressure would be essential, particularly during the parturition period between February and May. Full article

As major bleaching continues to ravage reefs worldwide, there is an urgent need for active coral restoration. However, the high cost of such a project is inhibitive for many countries. Here, we introduce a cost-effective design for Acropora robusta and Acroporavalenciennesi coral gardening through fragmentation and direct transplantation. Implemented off Boundary Island, Hainan Province, China, the project demonstrated high coral survival rates (>94%) at a reduced cost of USD 2.50 per coral after 246 days, besides exhibiting an efficient outplanting rate at 30 coral h⁻¹ person⁻¹. Growth monitoring suggested that the transplanted Acropora spp. follow an exponential growth model over time. Initial fragment size did not seem to affect the growth rate of outplanted Acropora spp., although a weak negative correlation was found at day 246 for A. robusta. Finally, the design used in this study employs detachable steel grid nurseries and is plastics-free, ensuring sustainability and adaptability to different reef conditions, and thus providing a promising strategy for affordable coral reef restoration. Full article

Nodal tides, which follow an 18.6-year cycle, influence tidal variations at any given location in the ocean. Conventional nodal tide theory neglects land effects and topological change. Due to the complex seabed topography around Taiwan waters, the purpose of this paper is to use the long-term tidal data of six stations to discuss the effects of perigean and nodal tides on 20 constituents and to compare the results with previous theories. A modulation method is employed to fit the annual amplitude estimated by harmonic analysis (HA). The top four constituents of the fitted and theoretical values of nodal amplitude factor (AF) and phase factor (PF) are O₁, K₁, K₂, and Q₁. We find that perigean tides or second-order nodal tides considered in the fitting contribute to almost identical performance. The linear time change considered in the AF fitting has better fitting than the mean water level involved. Full article

Marine oil spills cause significant environmental damage worldwide. Marine radar imagery is used for oil spill detection. However, the rapid attenuation of backscatter intensity with increasing distance limits detectable coverage. A multi-stage image enhancement framework integrating background clutter fitting subtraction, Multi-Scale Retinex, and Gamma correction is proposed. Experimental results using marine radar images sampled in the oil spill incident in Dalian 2010 are used to demonstrate the significant improvements. Compared to Contrast-Limited Adaptive Histogram Equalization and Partially Overlapped Sub-block Histogram Equalization, the proposed method enhances image contrast by 24.01% and improves the measurement of enhancement by entropy by 17.11%. Quantitative analysis demonstrates 95% oil spill detection accuracy through visual interpretation, while significantly expanding detectable coverage for oil extraction. Full article

The global shipping industry is evolving towards deep integration of digital transformation, intelligent upgrading, and green development. Meanwhile, recent geopolitical shifts have introduced heightened uncertainties into international shipping, compounding the challenges and escalating the demands for weather routing services for ocean-going ships. This paper provides a systematic review and expert perspective on China’s current status and key challenges in ocean-going weather routing services. Based on operational insights from China’s national meteorological service synthesized with a review of current trends and the literature, it further explores the future development of China’s ocean-going weather routing services and technologies from multiple dimensions: enhancing maritime weather observation capabilities, developing advanced weather routing service models, upgrading autonomous and controllable global satellite communication systems, promoting intelligent navigation technologies to facilitate shipping’s low-carbon transition, and expanding meteorological support capabilities for Arctic shipping routes. The analysis identifies critical gaps and proposes strategic directions, offering a unique contribution to understanding the trajectory of weather routing services within China’s specific national context from the perspective of its primary national service provider. Full article