Coasts, Volume 5, Issue 3 (September 2025) – 9 articles

Bivalves such as oysters rely on aragonite and calcite for shell formation via the biomineralization of calcium carbonate. Ocean acidification reduces carbonate ion availability, compromising shell growth and inducing dissolution under undersaturated conditions ($\Omega$ < 1). This study assessed the aragonite and calcite saturation state ($\Omega$) as a proxy for evaluating habitat suitability for oyster aquaculture and restoration. Temperature, salinity, pH, and total alkalinity were monitored across multiple sites and used to calculate the aragonite and calcite saturation state via the Seacarb package. Calcium hardness and dissolved oxygen were also measured to evaluate compliance with hatchery water quality standards. Results indicated temporal and spatial fluctuations in saturation states, with frequent undersaturation during cooler months. Spearman correlation analyses demonstrated significant positive relationships between temperature and salinity (p = 0.46), between pH and aragonite saturation state (p = 0.72), and between alkalinity and aragonite saturation state (p = 0.51). These findings highlight the importance of carbonate chemistry variability and seasonal drivers in determining the suitability of sites for oyster cultivation and restoration under changing environmental conditions. Full article

This study aimed to evaluate how tidal modulation influences breaking waves on a macrotidal beach along the Amazonian coast under varying climatic conditions. The study utilized medium-term data (2006–2018) from national and international institutions and short-term data (2012–2014) from in situ measurements at Ajuruteua Beach. Offshore winds and waves, predominantly from the northeast, were influenced by severe storms associated with La Niña and El Niño events. During these periods, wave heights exceeded 5 m, with wave periods ranging from 12 to 20 s. Tidal fluctuations (typically 5.0–6.0 m) modulated nearshore wave heights and periods, with variations determined by offshore conditions and climatic influences. Wave heights decreased from 2–5 m offshore to 1–2 m nearshore. At low tide, sandbanks dissipated wave energy, resulting in significantly smaller breaking waves (0.1–0.5 m) compared with high tide (1–1.8 m). The northern part of Ajuruteua Beach experienced a progressive retreat, with a total area loss of 0.15 km² and a shoreline retreat of 0.360 km between 2007 and 2021. The combination of high hydrodynamic energy and unregulated development led to the destruction of 43 buildings between 2007 and 2013 and an additional 44 houses between 2013 and 2021 within the intertidal zone. Moreover, the absence of coastal management strategies has exacerbated erosion, underscoring the urgent need for planning and regulatory frameworks. Based on the findings of this study, it is recommended that land use be regulated and both short- and long-term physical processes be systematically integrated into future coastal protection planning. Full article

The city of Acapulco was impacted by two near-consecutive hurricanes. On 25 October 2023, Hurricane Otis made landfall, reaching the highest Category 5 storm on the Saffir–Simpson scale, causing extensive coastal destruction due to extreme winds and waves. Nearly one year later (23 September 2024), Hurricane John—a Category 2 storm—caused severe flooding despite its lower intensity, primarily due to its unusual trajectory and prolonged rainfall. Digital shoreline analysis of PlanetScope images (captured one month before and after Hurricane Otis) revealed that the southern coast of Acapulco, specifically Zona Diamante—where the major seafront hotels are located—experienced substantial shoreline erosion (94 ha) and damage. In the northwestern section of the study area, the Coyuca Bar experienced the most dramatic geomorphological change in surface area. This was primarily due to the complete disappearance of the bar on October 26, which resulted in a shoreline retreat of 85 m immediately after the passage of Hurricane Otis. Sentinel-1 Synthetic Aperture Radar (SAR) showed that Hurricane John inundated 2385 ha, four times greater than Hurricane Otis’s flooding (567 ha). The retrofitted QGIS methodology demonstrated high reliability when compared to limited in situ local reports. Given the increased frequency of intense hurricanes, these methods and findings will be relevant in other coastal areas for monitoring and managing local communities affected by severe climate events. Full article

Heavy metal pollution from human activities is an increasing environmental concern. This study investigates the concentrations of Cu, Pb, Zn, Cd, Hg, and As in the coastal seawater offshore of Beihai, Guangxi, in April 2021, and explores their relationships with dissolved inorganic nitrogen, phosphate, and salinity. Our results reveal higher heavy metal concentrations in the northern nearshore waters and lower levels in southern offshore areas, with surface waters generally exhibiting greater enrichment than bottom waters. Surface concentrations show a decreasing trend from the northeast to the southwest, likely influenced by prevailing northeast monsoon winds. While bottom water concentrations decline from the northwest to the southeast, which indicates the influence of riverine runoff, particularly from the Qinzhou Bay estuary. Heavy metal levels in southern Beihai waters are comparable to those in the Beibu Gulf, except for Hg and Zn, which are significantly higher in the water of the Beibu Gulf. Notably, heavy metal concentrations in both Beihai and Beibu Gulf remain considerably lower than those observed in the coastal waters of Guangdong. Overall, Beihai’s coastal seawater meets China’s Class I quality standards. Nonetheless, continued monitoring is essential, especially of the potential ecological impacts of Hg and Zn on marine life. Full article

Recreational diving depends on healthy marine ecosystems, yet it can harm biodiversity through species displacement and habitat damage. Bombinhas, a biodiverse diving hotspot in southern Brazil, faces growing threats from human activity and climate change. This study assessed the ecological structure of Sepultura Beach (2018) for potential diving trails, comparing it with historical data from Porto Belo Island. Using visual censuses, transects, and photo-quadrats across six sampling campaigns, researchers documented 2419 organisms from five zoological groups, identifying 14 dominant species, including Haemulon aurolineatum and Diplodus argenteus. Cluster analysis revealed three ecological zones, with higher biodiversity at the site’s edges (Groups 1 and 3), but these areas also hosted endangered species like Epinephelus marginatus, complicating trail planning. A major concern was the widespread bleaching of the zoanthid Palythoa caribaeorum, a key ecosystem engineer, likely due to rising sea temperatures (+1.68 °C from 1961–2018) and declining chlorophyll-a levels post-2015. Comparisons with past data showed a 0.33 °C increase in species’ thermal preferences over 17 years, alongside lower trophic levels and greater ecological vulnerability, indicating tropicalization from the expanding Brazil Current. While Sepultura Beach’s biodiversity supports diving tourism, conservation efforts must address coral bleaching and endangered species protection. Long-term monitoring is crucial to track warming impacts, and adaptive management is needed for sustainable trail development. The study highlights the urgent need to balance ecotourism with climate resilience in subtropical marine ecosystems. Full article

Coastal climate processes that affect landscape hydrology and beach dynamics are studied using local and remote data sets near Cape Vidal (28.12° S, 32.55° E). The sporadic intra-seasonal pulsing of coastal runoff, vegetation, and winds is analyzed to understand sediment inputs and transport by near-shore wind-waves and currents. River-borne sediments, eroded coral substrates, and reworked beach sand are mobilized by frequent storms. Surf-zone currents ~0.4 m/s instill the northward transport of ~6 10⁵ kg/yr/m. An analysis of the mean annual cycle over the period of 1997–2024 indicates a crest of rainfall over the Umfolozi catchment during summer (Oct–Mar), whereas coastal suspended sediment, based on satellite red-band reflectivity, rises in winter (Apr–Sep) due to a deeper mixed layer and larger northward wave heights. Sediment input to the beaches near Cape Vidal exhibit a 3–6-year cycle of southeasterly waves and rainy weather associated with cool La Nina tropical sea temperatures. Beachfront sand dunes are wind-swept and release sediment at ~10³ m³/yr/m, which builds tall back-dunes and helps replenish the shoreline, especially during anticyclonic dry spells. A wind event in Nov 2018 is analyzed to quantify aeolian transport, and a flood in Jan–Feb 2025 is studied for river plumes that meet with stormy seas. Management efforts to limit development and recreational access have contributed to a sustainable coastal environment despite rising tides and inland temperatures. Full article

Today, coastal storms represent one of the most formidable environmental challenges, causing significant impacts on coastal communities. This situation underscores both the importance and urgency of studying storms and their characterization. This study proposes an innovative approach combining Principal Component Analysis (PCA) and machine learning (Classification and Regression Trees, CART) to characterize and distinguish damaging storms from non-damaging ones along the coast of Dakar, Senegal. The analysis revealed that among several hydrometeorological variables studied (wave height, period, direction, runup, wave energy, sea level anomaly, tide, etc.), the variables SLA and tide play a central role in the occurrence of damage, although they are weakly correlated with the others. By cross-analyzing these variables, critical thresholds were established, such as Tide > 0.53 m combined with SLA ≥ 0.061 m, Tide > 0.53 m and ECWL ≥ 1.3 m, as well as Runup ≤ 0.64 m associated with a high wave period (Tp), allowing accurate differentiation of potentially damaging storms. The CART method validated these results and identified three key combinations: (1) Tide–SLA, where no damage is observed if Tide < 0.53 m, and damage occurs beyond this threshold when SLA ≥ 0.061 m; (2) Tide–ECWL, where storms are damaging if Tide > 0.53 m and ECWL ≥ 1.3 m; (3) Runup–Tp, where storms are damaging if Runup ≤ 0.64 m or if Runup > 0.82 m with Tp ≥ 16 s. These results constitute the first application of machine learning for storm classification on the Senegalese coast, providing a novel quantitative foundation for better understanding the hydrodynamic conditions associated with damaging storms. The findings of this study could be valuable for risk management and the development of early warning systems Full article

Coastal wetlands play important environmental roles. However, their hydrogeomorphological dynamics remain poorly understood under scenarios of extreme climate events. The aim of this study was to characterize the temporal and spatial variability of hydrogeomorphological attributes (vegetation, water, and soil) in the wetlands of Lagoa do Peixe National Park, Brazil. The methodology involved applying Principal Component Analysis (PCA) in both temporal (T) and spatial (S) modes, decomposing spectral indices for each attribute to identify variability patterns. The results revealed that vegetation and water are strongly correlated with seasonal dynamics influenced by ENSO (El Niño/La Niña) events. Soils reflected their textural characteristics, with a distinct temporal response to the water balance. PCA proved to be a useful tool for synthesizing large volumes of multitemporal data and detecting dominant variability patterns. It highlighted the Lagoon Terraces and the Lagoon Fringe, where low slopes amplified hydrological variations. Temporal variability was more responsive to climate extremes, with implications for ecosystem conservation, while spatial variability was modulated by geomorphology. Full article

Intraspecific variations in the morphological traits of juveniles and adults of the Brazilian silverside, Atherinella brasiliensis, from three estuarine habitats were studied to understanding whether their morphology interacts with their dietary composition and habitat structure. For each individual, fourteen morphological measurements and eight functional traits were recorded related to food acquisition and locomotion. The highest abundance of A. brasiliensis was recorded in mudflats, which were often associated with a greater number of juveniles. Overall, 392 A. brasiliensis stomachs were examined, and their diet comprised mainly zooplankton organisms, followed by insects and benthic crustaceans. Among the morphological measures, our data revealed that in vegetated habitats (seagrass and riparian vegetation), individuals showed a higher oral gape surface and caudal peduncle and fed predominately on epibiotic or benthic fauna, while for individuals that had bigger eyes in unvegetated habitats (mudflat), this facilitated the ingestion of zooplankton and diatoms. Furthermore, a greater relative body height recorded in unvegetated habitats enhanced swimming performance and was linked to the effects of the lowest habitat structure. The results highlight the significant effects of morphological variation on juvenile and adult food acquisition and swimming ability. Full article