Nitrogen cycle dynamics in oxygen-deficient zones require special attention due to their close relationship with global climate change on various time scales. Sinking particulate organic matter is a fundamental part of the connection between physicochemical processes in the water column and the sedimentary record, from which relevant information on biogeochemical cycles and their modeled future implications are obtained. In the present study, a joint reanalysis of exported productivity tracers (i.e., organic carbon, biogenic opal, and calcium carbonate), organic matter sources (i.e., C:N ratio and stable carbon isotopes), and water column denitrification (i.e., stable nitrogen isotopes in particulate matter) was carried out in the Alfonso basin. This study aims to investigate the relationships among exported productivity, water-column denitrification, and the nitrogen isotopic composition of sinking organic matter in an oxygen-deficient basin of the Gulf of California to improve our understanding of biogeochemical processes governing cycling nitrogen and isotopic variability in oxygen-deficient zones. The productivity tracers associated with siliceous plankton were not seasonal, contrasting with calcareous plankton that showed minimum values under high primary productivity and organic carbon flux conditions in the Alfonso basin. Calcareous plankton increased its relative contribution concerning siliceous plankton in the study period (2002 to 2005). The dominant sinking organic matter source was of marine origin, with no contribution from terrestrial organic matter. The variability of δ15N values, with minima from January to May and September to December and maxima from June to August 2002 to 2005, was interrupted by maxima from January to May 2003 and 2004. This intense water column denitrification period was associated with organic carbon and biogenic opal flux, suggesting nutrient availability due to the persistence of a cyclonic gyre and a high demand for dissolved oxygen consumption derived from exported productivity associated with the opal plankton community. The nitrogen isotopic differential of sinking particulate matter (10.5‰) concerning surface sediment (11.1‰) was 0.6‰ for the Alfonso basin, lower than the 1.7‰ for the Guaymas basin and 2.3‰ for the global average. This indicated that processes in the water column and early diagenesis in the sediment slightly affected this study region. They more accurately reflected the oceanographic processes that control and connect the water column to the sediment in this oxygen-deficient zone of the transitional Mexican Pacific.
This study quantifies and characterises the seasonal ingestion of anthropogenic items (AIs), including microplastics, by round sardinella (Sardinella aurita) from the coast off Barcelona (NW Mediterranean Sea), and assesses their potential impact on fish health through body condition indices and histological assessment. For this purpose, 100 specimens were seasonally captured aboard commercial purse seiner fishing vessels between 2022 and 2023. Present findings indicate that cellulosic fibres were predominant throughout the year (representing 73.56% of all AIs ingested). Fish ingested a mean of 2.27 (SD = 2.13) items/individual, with winter 2023 being the season with the highest abundance of AIs ingested and summer of the same year the lowest (GLM, p< 0.05). Seasonal variations in AIs ingestion were consistent with environmental (i.e. rainfall rates, thermocline stratification) and biological (i.e. feeding behaviour, reproduction) factors. Thus, higher AIs ingestion in winter coincided with increased feeding intensity and filter-feeding behaviour, whereas lower AIs ingestion in summer of 2023 was associated with reduced feeding activity, particulate-feeding behaviour and lower rainfall and riverine inputs. None of the histopathological alterations observed could be attributed to AIs exposure, even during the season with the highest ingestion rates. Studies with a seasonal approach, as the present case, are crucial for identifying periods of high AIs exposure and for accurately assessing the potential risks to fish condition, providing valuable insights into the temporal dynamics of AIs ingestion in marine species.
Copepods belonging to the order Cyclopoida are important components of tropical coastal zooplankton communities and play a significant role in aquatic food webs due to their ecological adaptability and widespread distribution. The present study investigated the diversity, density, and spatiotemporal distribution of species belonging to the genera Apocyclops and Oithona/Dioithona across different seasons and environmental gradients in the Kovalam coastal region of the Bay of Bengal, India, and reports significant spatial and seasonal variability in these taxa for the first time. Samples were collected from three ecologically distinct zones—coastal (KCR), estuarine (KER), and backwater (KBW)—during the summer, southwest monsoon, northeast monsoon, and post-monsoon periods. Zooplankton samples were collected, preserved, and taxonomically identified to the species level using standard methods. Concurrently, physicochemical parameters, including sea surface temperature (SST), pH, salinity, dissolved oxygen (DO), nitrate, nitrite, inorganic phosphate, ammonia, and chlorophyll-a (Chl-a), were analyzed. Statistical analyses included Pearson correlation analysis, Principal Component Analysis (PCA), and Canonical Correspondence Analysis (CCA). The results revealed significant seasonal and spatial variations in physicochemical parameters and copepod assemblages (p < 0.05). Correlation analysis identified significant associations between copepod populations and environmental variables, while PCA highlighted substantial seasonal and spatial variability in environmental conditions. CCA further demonstrated the influence of environmental gradients on the distribution and community structure of Apocyclops and Oithona/Dioithona species across the Kovalam region. Overall, the findings indicate that Apocyclops royi and A. dengizicus exhibit broad ecological tolerance and adaptability to fluctuating environmental conditions, providing valuable baseline information for understanding copepod community dynamics in tropical coastal ecosystems and supporting future ecological and applied investigations.
Extreme wind stress and pressure anomalies associated with typhoons can rapidly disrupt nearshore hydrodynamics, strongly influencing the evolution and stability of coastal upwelling systems. The Taiwan Strait, characterized by monsoon-driven upwelling and frequent typhoon passages, provides an ideal setting to explore these interactions. Utilizing long-term Acoustic Wave and Current Profiler observations and diurnal full-tidal-cycle Conductivity–Temperature–Depth observations near Pingtan Island in summer 2013, combined with meteorological and reanalysis data, this study investigates the spatiotemporal evolution of current structure, temperature field, and upwelling intensity during Typhoons Soulik and Trami. These observations reveal the transition of the coastal upwelling system from stable maintenance to typhoon-induced disruption and subsequent rapid reconstruction. Based on these results, we propose a conceptual “destruction–reconstruction” model for the typhoon-influenced upwelling system. During typhoon forcing, typhoon-induced winds and cross-shore current disturbances drastically suppress the upwelling of cold nearshore water. After the typhoon passage, the recovery of background southwesterly winds and the geostrophic adjustment enable upwelling reestablishment within 3–5 days. Wavelet coherence analysis shows that local wind stress is the dominant driving force, with surface currents responding instantaneously and bottom currents lagging by about 1/24 of a cycle. These findings highlight the high sensitivity and regional specificity of shallow-shelf upwelling to extreme weather events, providing valuable scientific evidence for regional marine environmental impact assessment.
Nitrous oxide (N2O) is a potent greenhouse gas and a significant contributor to stratospheric ozone depletion. The ocean accounts for one-third of natural N2O emissions to the atmosphere; however, substantial uncertainties remain regarding their spatial distribution and magnitude. This study examined seasonal variability in dissolved N2O and associated biogeochemical parameters in the southeastern Arabian Sea (SEAS) to elucidate production pathways and sea-to-air fluxes by discrete measurements during the southwest monsoon (SWM) and northeast monsoon (NEM) cruises covering four coastal cross-shore transects (Kollam, Alappuzha, Kochi, and Mangalore). The southwest monsoon (SWM) period was characterised by pronounced hydrographic changes, driven by coastal upwelling, resulting in shelf hypoxia and enhanced accumulation of N2O in the water column. A significant negative correlation between N2O and Ekman transport during the SWM indicates that upwelling is the key process controlling N2O accumulation in shelf waters. Conversely, N2O concentration was relatively low during the northeast monsoon (NEM). Strong positive correlations of N2O with apparent oxygen utilisation (AOU), the sum of dissolved nitrate and nitrite (NO2-+NO3-), and excess N2O (ΔN2O) suggest that nitrification is the primary N2O production pathway. However, during the SWM, a significant negative correlation between N2O and dissolved nitrite under hypoxic conditions (dissolved oxygen < 62 μM), coupled with relatively high levels of labile organic matter on the inner shelf, suggests the possibility of nitrifier denitrification. The sea-to-air N2O fluxes exhibited substantial seasonal variability, ranging from 0.10 to 48.2 μM m−2 d−1 (9.1 ± 11.0), with consistently high N2O supersaturation. The significant efflux of N2O during both seasons suggests that the SEAS acts as a persistent regional source of atmospheric N2O. The estimated annual mean flux densities from the SEAS yield a net annual source of 8.0 x 10-3 Tg-N2O yr−1 (5.0 x 10-3 Tg-N yr−1). Overall, this study demonstrates that seasonal upwelling, oxygen depletion, and nitrogen cycling strongly regulate N2O production and emission in the SEAS, with nitrification as the dominant pathway and potential contributions from nitrifier denitrification under hypoxic conditions. The consistently high N2O supersaturation and substantial sea-to-air fluxes establish the SEAS as a persistent regional source of atmospheric N2O, highlighting its importance to regional and global oceanic N2O budgets.
We present spatial and temporal measurements of surface water nitrous oxide (N2O) and methane (CH4) concentrations around Vancouver Island, BC. Using an automated, high-frequency measurement system, we conducted a spatial survey along the island's east and west coasts, and obtained a one-week time series from Barkley Sound on the west coast. These measurements allowed us to assess the influence of wind-driven circulation and tidal forcing on surface water hydrography and N2O and CH4 distributions. Across the spatial survey, surface water N2O ranged from 69% to 133% saturation, while CH4 ranged from 83% to 936% saturation. Elevated concentrations of both gases were associated with regions of intense tidal mixing in the Strait of Juan de Fuca and Johnstone Strait. In contrast, waters along the west coast of Vancouver Island exhibited greater mixed layer stratification, limiting vertical inputs of N2O and CH4 to the mixed layer. Elevated surface water concentrations in this region are more likely attributable to horizontal transport via the Vancouver Island Coastal Current. High-frequency measurements in Barkley Sound demonstrated significant temporal variability over a one-week period, with N2O saturation ranging from 76% to 177%, and CH4 saturation ranging from 163% to 556%. This variability was associated with tidal forcing and diel changes in surface winds, which drove lateral transport of different source water masses with distinct gas signatures. Our work highlights the utility of high frequency measurements to capture variability in N2O and CH4 concentrations in a dynamic coastal region, and provides insights into the interacting physical processes driving this variability.
The increase in continental shelf hypoxia (oxygen concentration <2 mg l−1) throughout the world is primarily a result of land-sourced nutrient enrichment. The well-recognized consequences to food webs, commercial fisheries, and economies vary according to the duration, frequency and severity of oxygen depletion. The increased nutrient loading of the Mississippi River into the northern Gulf of Mexico (a.k.a., ‘Gulf of America’) in the mid-1970s causes the formation of the largest hypoxic zone in the western Atlantic Ocean. The reported data on oxygen monitoring in this region is largely from biweekly to annual sampling, and so we infer how variable and consistent hypoxia is during years, seasons or months, but not among days.We increased knowledge of the temporal variability in the area by deploying water quality meters on the inner Louisiana continental shelf to record, at 15-min intervals, dissolved oxygen, percent O2 saturation, temperature, salinity, and conductivity at two locations in 20-m water depth from 1989 to 2010. Data were collected over 5975 days from 137 of 160 deployments lasting an average of 43.6 ± 2.2 days. Hypoxia was observed in all months, but not all months in all years had hypoxia. Hypoxia was present on 34% of the deployment days; 63% of those hypoxic days had oxygen concentrations <1 mg l−1, and 35% of the hypoxic days had oxygen concentration approximately = 0 mg l−1. The peak occurrence in the percentage of days that were hypoxic coincided with the annual hypoxia cruise occurring in the last week of July and the lowest percentage was in November to February. Hypoxia persisted for at least 30 d or more in summer, with shorter periods of 2 to 12 d following reoxygenation from water column mixing or advection of a higher dissolved oxygen water mass to the area.It can be assumed that dissolved oxygen concentrations are extremely low over a broad area of the inner continental shelf west of the Mississippi River over extended periods and are sufficiently low to stress or kill demersal and infaunal organisms.
Niche theory proposes that the partitioning of different niche dimensions is a mechanism that enables the coexistence of co-occurring species. In this context, the present study aimed to investigate the trophic ecology of five sea urchin species that cohabit a region characterized by intense upwelling events in southeastern Brazil, using isotopic analyses and gut content data. Trophic partitioning was identified among the less abundant species inhabiting the rocky shore. Eucidaris tribuloides exhibited a narrower isotopic niche width and a diet predominantly composed of crustose coralline algae (CCA) and barnacles. Lytechinus variegatus showed a broad isotopic niche amplitude, indicating the assimilation of multiple trophic resource sources. Gut content analysis corroborated these findings, revealing a substantial presence of various algal taxa, although sediment predominated. This feeding habit appears to be linked to its use of a microhabitat (interface between the rocky shore and unconsolidated substrate) which differentiates it from the other species. Arbacia lixula had a small sample size. However, the results are consistent with previous literature, indicating a diet based largely on CCA and a relatively high consumption of animal material. The most abundant species in the region, Echinometra lucunter and Paracentrotus gaimardi, exhibited high trophic overlap, both isotopic niche area and gut content. This overlap appears to result from the strong trophic generalism of both species, which exploit and consume available resources in a manner that likely minimizes competitive processes due to food availability and broad dietary plasticity.
Seamounts are key biodiversity hotspots for marine ecosystems but highly vulnerable to human impacts, highlighting the urgent need for effective conservation measures. Their remoteness and deep habitats, however, challenge the implementation of the long-term monitoring programs necessary to quantify the protection effect for reliable conservation benchmarks. This study evaluates the potential of passive acoustic monitoring (PAM) combined with ecoacoustic indices as a practical and cost-effective tool for ecological assessment of seamounts. We use this approach to analyze the presence and diversity of fish sounds across four mid-north Atlantic seamounts of contrasting summit depths. Results showed a greater presence and diversity of fish sounds in shallower seamounts, likely reflecting higher habitat complexity and species richness. Acoustic activity also exhibited clear diel patterns, with the occurrence of several sound types showing marked differences between day and night, suggesting temporal variation in fish behaviour and habitat use. Variations in fish sound diversity was ascertained and cross-validated by three ecoacoustic indices: Acoustic Complexity Index (ACI), Entropy (H), and Sound Pressure Level (SPL), and supported by in-situ evidence from underwater visual census at the summits of shallow seamounts. These findings support PAM as a viable and valuable tool for assessing and monitoring fish biodiversity in seamounts, providing essential data at relatively low cost in support of management policies for these fragile ecosystems.
Seagrass ecosystems are exposed to natural environmental variability and climate-modulated extremes that can interact with anthropogenic pressures and accelerate ecological decline. This systematic review synthesized 60 peer-reviewed publications published between 2010 and 2025 to examine how temperature and marine heatwaves, ocean acidification and pH, salinity, drought and flooding, hydrodynamics, sea-level change, turbidity and light limitation, sediment conditions, and oxygen availability affect seagrass ecosystems. Thermal variables were the most frequently investigated category; however, publication frequency was treated as an indicator of research effort rather than a ranking of ecological importance. Experimental and field evidence showed that warming and marine heatwaves often reduce growth, photosynthetic performance, biomass, and recovery, although outcomes depend on species, exposure duration, light, nutrients, and prior stress history. Evidence for warming–acidification interactions was contrasting: acidification impaired plant performance during thermal stress in some studies, whereas enhanced inorganic carbon availability supported biomass or carbon metabolism in others. Most multi-stressor experiments evaluated only two drivers, and relatively few examined three or more simultaneous or successive stressors. The most commonly reported ecosystem consequences included habitat contraction, canopy and biomass loss, physiological impairment, altered carbon and nutrient cycling, sediment destabilization, community reorganization, and reduced resilience. Geographic evidence remained concentrated in Europe, the Mediterranean, Australia, China, and North America. Indonesia and much of Southeast Asia were comparatively underrepresented despite high seagrass diversity and documented declines associated with interacting environmental change. Future research should prioritize standardized long-term monitoring and multifactorial experiments that distinguish research frequency from ecological effect and support regionally calibrated risk assessment.
Global sea level history capturing the rise in sea levels since the Last Glacial Maximum, about 20,000 years ago, is generally well-resolved to the scale of millennia. However, when examining this record at a finer scale of hundreds of years, discrepancies remain particularly as it pertains to the timing of accelerated periods of sea level rise caused by outsized glacial meltwater events (or pulses). We present here an inventory of sea level indicators in the form of submerged paleoshorelines that have been identified across the west Florida shelf for the depth range of 45-90 m. The region's well-preserved carbonate sedimentary record coupled with its tectonic stability provides a setting in which sea level changes since ∼20 ka have been recorded to a remarkable degree of resolution in the form of paleoshoreline features that formed during still stands or slowdowns of sea level rise. In total, 27 paleoshorelines were identified based on geomorphic identifiers and used as paleo-sea level indicators to put forth a hypothetical scenario for sea level history between 14.2 ka – 11.2 ka. In all, our findings describe 15 previously unidentified features and present new constraints to the sequence of meltwater pulses in the Gulf of Mexico, which challenge current estimates. We suggest that in the periods preceding and in between meltwater pulses (MWP)-1A and -1B, rates of sea level rise were slow enough for the formation of shorelines to occur; followed by periods of rapid sea level rise (MWP-1A and -1B) which were sufficiently high enough to drown and preserve these shorelines in place.
This study examines the hydrodynamic interactions and estuarine-shelf exchange mechanisms at the two primary mouths of the Paranaguá Estuarine Complex (PEC), a multi-mouth system in Southern Brazil. Through intensive synoptic sampling, we define the hierarchy of controls governing flow partitioning and transport across tidal and seasonal scales. Our results reveal that while tidal forcing sets the short-term energetic baseline, the net estuarine flux is fundamentally modulated by the interplay between seasonal buoyancy and episodic shelf-driven meteorological surges. We identify a seasonal functional reversal: in summer, high riverine discharge enhances baroclinic-driven gravitational circulation, favoring seaward export primarily through the southern mouth. In contrast, during winter, the decay of buoyancy allows shelf-driven pressure gradients to override internal tidal asymmetry, shifting the system toward a net import state. This functional inversion is dynamically supported by the southern mouth's geometry, which acts as a ‘dynamic valve’ that facilitates rotational lateral shear and landward intrusion under Coriolis influence. We conclude that the PEC does not function as a passive drainage conduit but as a selective filter for shelf-borne materials, where mouth geomorphology and shelf connectivity redefine the system's role from a source to a sink of coastal waters. These findings provide a transferable model for mid-latitude multi-mouth estuaries, highlighting how sub-tidal forcing can episodically override astronomical tidal signals in systems with shallow-water tidal distortion.
Coastal lagoons support high biodiversity and provide many ecosystem services but are under threat from changes in freshwater flow due to climate change and direct anthropogenic actions. Understanding how phytoplankton community structure responds to flow-induced changes in environmental conditions is critical for assessing the impact of climate change on ecosystem function. This study investigated patterns in phytoplankton abundance and diversity across a wide range of environmental conditions, from extreme dry (drought) to extreme wet (flood), by analysing a 16-year dataset (2007–2023) in the Coorong, South Australia, a Ramsar-listed shallow coastal lagoon. The Coorong stretches over 100 km long and functions as a reverse estuary, with a salinity gradient from fresh to brackish water near the entrance to the ocean to hypersaline in its southern reaches. Multiple statistical analysis techniques, including correlations, analysis of principal coordinates, biota and environmental matching, distance-based linear modelling and generalised additive modelling identified salinity and water level as the principal drivers of change in the abundance and diversity of the phytoplankton community, with distance from the ocean inlet and water temperature also significant, but to a lesser degree. Phytoplankton communities were characterised by salinity-derived habitats, from ‘freshwater’ H1 to ‘extreme hypersaline’ H6. Cyanoprokaryotes and chlorophytes dominated in H1, whereas Diatoms and Dinoflagellates increased their relative abundance with increasing salinity, ultimately dominating hypersaline H6. The most equal representation of all groups occurred in estuarine H2. This work supports the development of a habitat-based predictive framework for coastal lagoon management to adapt to climate change.
In estuarine environments, navigational channels require recurrent maintenance dredging (MD), involving large volumes of sediment. Despite its importance, the medium-term (∼10 years) impact of MD on hydro-morpho-sedimentary dynamics in sandy-muddy estuaries remains poorly documented. This study addresses this issue using the Seine Estuary (France) as a well-documented case.We used a 3D hydro-morpho-dynamic model, coupled with a wave model, to simulate sand-mud dynamics, accounting for MD and dumping as active processes. The simulated disposal-site stability is consistent with observations, and numerical sediment tracing clearly illustrates the contrasting behaviour of sand and mud on disposal sites.To assess the effects of MD on turbidity, sediment fluxes, and morphological evolution, three scenarios were simulated over 9 years: the reference scenario, which involved dredging and disposal as currently practised; a dredging-without-disposal scenario, to isolate the effect of dumping; and a no-dredging scenario, to isolate the effect of dredging. The estuarine turbidity maximum (ETM) mass shows only a slight increase with dredging, despite an increasing contribution of previously dredged material to its total mass, which reaches about 14% after 9 years. This paradox is explained by the cumulative remobilisation of sediment, which differs only slightly between scenarios with and without dredging, with the remobilised mass greatly exceeding the ETM mass. Morphological evolution is similar across scenarios, except near dredging and disposal sites, where dredging reduces siltation and alters local morphological evolution. MD enhances sediment fluxes and, in particular, reverses the sediment budget, shifting the system from net infilling to net export. Overall, the results demonstrate the potential control exerted by sediment management strategies on long-term estuarine evolution and resilience to climate change.
A challenge with flood mitigation is that the water kept out of one floodplain can be redistributed to other locations. To date, however, the mechanisms and dynamics of coastal flood attenuation and induced flooding are under-researched. In this study, we analyze a 100-year coastal flood for Jamaica Bay, New York, an urbanized bay characterized by near standing-wave tides that provides a relatively simple case study of flood mitigation dynamics. We apply hydrodynamic modelling for detailed quantification of the effects of flood mitigation and idealized 1D semi-analytical modelling to investigate flood wave reflection processes. We evaluate three mitigation scenarios relative to a Control run, including floodplain and marsh restoration (nature-based solution NBS1), inlet narrowing and bay shallowing (NBS2), and a proposed surge barrier/levee system. Results show that the surge barrier system prevents all bay flooding but induces flood wave reflection and amplification outside the protected area. Three mechanisms enable NBS to reduce flooding while minimizing amplification and induced flooding: Flood wave dissipation, storage increases, and partial reflection. The 7.1% of additional bay area (storage) created with NBS1 leads to only a 1.4% reduction in flood wave amplitude in part due to a counteracting 3.1% increase in tidal prism. NBS2 causes an 18% increase in dissipation and a phase-shifted partial reflection that destructively interferes with the interior bay reflected flood wave, minimizing offshore increases in water level and leading to 54 times more flood prevention than initiation. Understanding these physical mechanisms behind NBS can help enable their application through design optimization.
As a typical semi-enclosed marginal sea, the dispersal and deposition mechanisms of terrestrial sediments in the Bohai Sea have long been debated. This study integrates major-element data from 146 stations and grain-size data from 447 surface sediment samples across the Bohai Sea. PCA and K-means clustering identify four geochemical provinces. Province I (Laizhou Bay and Bohai Bay) is enriched in Al-Ca-Mg-Ti-Fe, with a mean grain size of 6.5 Φ. Province IV (western Liaodong Bay) is also fine-grained (6.3 Φ), with similar elements enrichment but lower CaO. In contrast, Province II (central Bohai Sea, Bohai Strait, and near the Luanhe River estuary) is coarse-grained (4.5 Φ) and quartz-rich (average SiO2 content >70%). Province III (eastern Liaodong Bay) has 67% SiO2 contents and a mean grain size of 5.0 Φ. The TiO2/Al2O3 ratio records the relative influence of river-derived sediments, whereas the SiO2/Al2O3 ratio reflects quartz enrichment and hydrodynamic sorting. Gao-Collins grain-size trend indicates potential net transport pathways from the Bohai Strait toward the central Bohai Basin, Laizhou Bay, and eastern Liaodong Bay, alongside redistribution trends from Yellow River Delta toward the central basin. EOF analysis results clarify the multi-factor combined effect of sediment dispersion: Mode 1 (41.6%) captures the bathymetry-related hydrodynamic and the distance from the sediment sources; Mode 2 (19.7%) reflects the effect of grain size and mineral composition; Mode 3 (14.0%) records the signal of Yellow River carbonate and marine biogenic debris. The findings provide new insights for research on sediment source-to-sink processes and marine environmental alterations.
This paper presents experiments involving microplastic (MP) transport and retention by marine obstacles such as gravel in (non-breaking) irregular waves combined with currents. Seven different non-buoyant MP groups having different shapes, dimensions and relative densities in the range 1.06 - 1.36 are considered, both in the presence and absence of gravel patches having different coverage densities and lengths. The results show that the retention and transport behavior of MP particles are significantly influenced by both particle Dean number and shape. Particles with higher Dean numbers (i.e. lower settling velocities) have higher mobility and lower retention tendency. Moreover, increasing the gravel obstacle coverage density and length substantially enhances MP retention efficiency, particularly for particles with lower Dean numbers. In wave +/- current conditions, the retention efficiency was generally reduced compared to pure wave scenarios. In addition, extended gravel patches with high density effectively retained most particle groups. These findings emphasize the role of benthic structures in influencing MP accumulation and suggest that small-scale marine obstacles can act as significant MP sinks under certain hydrodynamic conditions.
Over the outer continental shelf in the Middle Atlantic Bight, intrusions of relatively salty water are often found both at mid depth during stratified seasons and near the bottom when stratification is weaker. The possibility that these intrusions are the result of instabilities of the shelfbreak front is pursued here using an idealized primitive equation numerical model. Results show that indeed realistic salty intrusions are generated and that their properties are not dissimilar to known climatologies: the model intrusions are most common in the stratified interior of the water column and become increasingly rare shoreward of the front. No simulated water column intrusions develop when the shelf water column is weakly stratified, consistent with observations. Modeled near-bottom intrusions, on the other hand, occur more frequently as stratification weakens. Sensitivity to other model conditions (such as bottom friction, bottom slope, rotation rate and frontal strength) are also explored. The model eddy salt fluxes onto the shelf, however, fall short of that required by a shelf salinity budget.
