Continental Shelf Research最新文献

Seasonal and neap-spring tidal changes in estuarine physical variables and their effect on the phytoplankton distribution in the macrotidal Tanintharyi River estuary (TRE), Myanmar were studied in 2019. During the dry season, the saltwater intrudes upstream while only until the mid-estuary during the wet season due to the hindering of saltwater by the strong river flow. The turbidity variations driven by enhanced mixing and neap-spring transition influenced the light required for phytoplankton growth during both seasons. The diversity index was higher (2.57–3.07) in the dry season with low evenness (0.64–0.79) compared to the low diversity index (2.16–2.73) with high evenness (0.75–0.90) in the wet season. The peak diatom density was observed under low turbidity (6 FTU) and a salinity of 25, and their distribution were correlated positively with salinity, mean light intensity in the mixed layer (I_m) and euphotic depth (Z_e) while negatively with light attenuation (K) and turbidity. Among diatoms species, Thalassionema frauenfeldii, Thalassionema nitzschioides and Pseudonitzschia seriata were found as dominant species (positively correlated with salinity and I_m whereas negatively with turbidity and K) during the dry season whereas Thalassiosira eccentrica and Coscinodiscus granii were dominant with low salinity in the wet season. The dinoflagellates were more sensitive to strong mixing condition than diatoms and their distribution were positively influenced by I_m and Z_e. Therefore, the neap-spring tidal and seasonal variations in saltwater intrusion, mixing, turbidity, and light availability are the major factors that maintained the phytoplankton diversity in the tropical TRE.

This study investigates six areas in a historically heavily trawled region of the southern Baltic Sea. Using acoustic geophysical mapping data and sediment cores from three field campaigns (2019, 2020, 2023), we evaluate and quantify the cumulative physical impacts from bottom trawling and the influence of seabed geology on mapped trawl tracks. The results are compared with fishing intensity data over three periods; 2012–2016, 2017–2019 and after the fishery closed. A correlation between fishing intensity and density of mapped trawl tracks exists in the soft sediments of the northern part of the area, while this link is weak in the less trawled southern part, where the seabed is characterized by more consolidated glacial clays and the high density of mapped trawl tracks reflects the preservation of tracks >8 years old. Four years after the closure of the fishery there were no signs of trawl-track degradation in any of the areas. In summary, mapped track densities alone are not a suitable measure of trawling intensity, considering the influence of seabed geology and the persistence of trawl tracks over time. Sediment deformation, observed by CT-scanning, indicates extensive remoulding and coarsening of the upper 20–40 cm of sediments in the trawled areas.

We employed idealized numerical experiments using Delft3D to investigate how the interactions of coalescing buoyant river plumes with mangrove vegetation affect sediment transport and deposition in the coastal zone. Our model system, based on the Firth of Thames in the North Island of New Zealand, is designed to represent two mangrove-lined rivers debouching into a coastal bay. Sediment transport patterns into the three adjacent mangrove forest regions (central, western, and eastern) were assessed by quantifying the sediment fluxes over a tidal cycle. The interactions between the buoyant river plumes resulted in a coalesced river plume that flowed along the central mangrove forest between the rivers. The dominant contributors to the total sediment transport into the mangrove forests varied with both the total and the relative flows through the two rivers. When both rivers were forced with their respective flows at the 50th percentile of observed flow, the analysis of sediment transport fluxes along the central forest revealed sediment deposition inside the mangrove forests lining the river mouth nearly four times greater than in the intermediate regions along the central forest; consistent with satellite imagery of the Firth of Thames. Winds of 5 m s⁻¹ modify the overall sediment transport patterns by deflecting the river plume in the direction of the wind. The magnitude of sediment flux is the highest (nearly 80% compared to the no-wind scenario) in the case of an easterly wind scenario as the alongshore transport is enhanced by Coriolis deflection and wind direction aligned with the alongshore flows of the larger plume.

Phytoplankton in the marine environment exudates part of the primary production as dissolved organic carbon (DOC) into the surrounding waters. The rate of exudation of DOC is depended on the hydrographic condition, nutrient availability, phytoplankton composition, and their size structure. To examine this, samples were collected from the coastal and offshore regions where different hydrographic conditions exists in the Bay of Bengal. The coastal waters were relatively low saline, rich in inorganic nutrients, high phytoplankton biomass (Chl-a) and primary production in the coastal compared to offshore regions. The photic zone integrated Chl-a displayed insignificant difference between coastal and offshore regions whereas higher photic zone integrated primary production was observed in the former than latter region. Dominance of microplankton was observed in the coastal waters associating with high inorganic nitrogen concentrations, in contrast, picoplankton dominated in the offshore region associating with low inorganic nitrogen but high organic nitrogen concentrations. Due to high surface-to-volume ratio of picoplankton, ∼45% of the primary production exudated as DOC in the offshore compared to the coastal region (∼12%) due to dominance of microplankton. The sum of primary production and DOC exudation, called total primary production, was almost equal in the coastal and offshore regions. The mean phytoplankton biomass normalized primary production (pB) in the Bay of Bengal was low (17 ± 8 mgC mgChl-a⁻¹ d⁻¹) compared to Arabian Sea (37 ± 5 mgC mgChl-a⁻¹ d⁻¹). In contrast pB estimated based on total primary production is close (30 ± 16 mgC mgChla⁻¹ d⁻¹) to that of the Arabian Sea (37 ± 5 mgC mgChl-a⁻¹ d⁻¹) suggesting that the Bay of Bengal is equally productive compared to that of Arabian Sea than hitherto hypothesized due to strong stratification and less nutrients input in the former basin. The low sinking carbon flux observed below the photic zone is attributed to the release of primary production as DOC. The released DOC from the phytoplankton may support heterotrophic activity as evidenced by the existence of strong net heterotrophy in the Bay of Bengal. This may lead to the time lag between primary and export productions. None of these processes were incorporated into the numerical models resulting in inaccurate simulations of carbon cycling in the northern Indian Ocean. Modifications in the models by incorporating these processes may improve model simulations for a better understanding the modifications in biogeochemical processes due to climate change in the Bay of Bengal.

The mud depositional area ("mud area") of the South Yellow Sea serves as a prominent carbon sink within the Eastern Continental Shelf of China, offering crucial insights into human activities and climate fluctuations and its relationship with carbon cycle. This study investigates variations in total organic carbon (TOC) and black carbon (BC) concentrations in South Yellow Sea sediments. By combining ²¹⁰Pb isotope analysis with grain-size profiling, the study distinguishes organic carbon origins, revealing the complex interplay between human activities and environmental shifts since the Industrial Revolution. The reasons for discrepancies between TOC and BC contents are analyzed, yielding the following results: (1) Sediment core QY-2 predominantly contains terrestrial-sourced organic carbon, primarily derived from the Yellow River and the Yangtze River. (2) Temporal fluctuations of BC in core QY-2 show distinct trends, periodic responses to human activities, particularly peaks in 1937 and 1945, linked to large-scale wars in China. (3) Relaxed carbon emission regulations in China, coupled with industrial growth spurred by the “reform and opening up” policy, led to a continuous rise in BC content from the mid-1960s to the 1980s, peaking in 1980. Subsequently, reduced BC values during the 1990s correlated with emission control policies and the shift from highly polluting domestic coal stoves to cleaner alternatives like liquefied petroleum gas or natural gas stoves. (4) BC content was influenced by the East Asian monsoon and Pacific Decadal Oscillation (PDO), with higher BC accumulation rates occurring in summer and depletion in winter. (5) Interestingly, changes in BC and TOC content exhibited a negative correlation. While grain size and material sources minimally influenced these discrepancies, the primary driver lay in the water's redox environment, impacting other TOC components and thereby causing variations in both TOC and BC content. This study of black carbon sources to sinks in the South Yellow Sea Mud Area holds significant implications for the broader Yellow Sea sedimentary system and provides support for understanding carbon cycle and marine environments.

Mangrove forests play a crucial role in the coastal environment. A plethora of research is focusing on mangroves’ role in climate mitigation through sequestering and storing CO₂. This study aims to assess the organic carbon (OC) stocks and carbon sequestration rate (CSR) of the largest stands of mangroves in the Red Sea Coast of Egypt, to evaluate their significance in climate mitigation strategies. Intensive sampling was conducted in mangrove stands of Safaga, Qulaan, and Hamata to investigate OC and sediment properties along depth intervals to 1 m deep. Additionally, OC stored in the above- and below-ground biomass was estimated using Landsat 9 OLI/TIRS satellite images. TOC content (∼0.50%) and stocks in the sediments (54.49 ± 31.71–86.00 ± 29.00 MgC/ha) were very low compared to the global average for mangrove stands, and comparable to the mangrove forest in the Middle East region. This can be attributed to the desert and semi-desert conditions, limited riverine input, and low precipitation rates. Moreover, the above-ground biomass, estimated from Normalized Difference Vegetation Index, exhibited extremely low OC storage. Therefore, the sediment acted as the main pool of organic carbon in these mangrove ecosystems. The research findings revealed that mangrove stands in Egypt exhibit low CSR, ranging from 11.94 to 18.02 g C m⁻² year⁻¹. Additionally, the study determined that the stable nature of mangrove stands in Egypt makes their annual CO₂ emissions negligible. Despite local studies suggesting otherwise, it is highly unlikely that mangroves in Egypt contribute to the climate mitigation related to CO₂ sequestration. Therefore, it is important to implement restoration plans to ensure the preservation of other ecological services provided by mangroves along the Red Sea Coast.

The growth of the now ubiquitous hypoxic zones found throughout the global coastal ocean are primarily a consequence of nutrient enrichment in surface waters increasing organic production that sinks into bottom waters where oxygen is depleted faster than it is replenished. Hypoxic zones may increase or decline in number because of future climate changes. Here we summarize the summertime variations of dissolved inorganic silicate (DSi), phosphate (DIP), nitrogen (DIN; nitrate + nitrite and ammonium) and ammonium concentrations in the bottom waters of the northern Gulf of Mexico continental shelf from 1985 to 2022. The concentrations of all three are strongly correlated to oxygen concentrations, but not in the same way. At zero oxygen concentration, the annual concentrations of DSi, DIP, and ammonium changed over 38 years at a rate of 1.6 % y⁻¹, 2.0 % y⁻¹ and -1.7 % y⁻¹, respectively. However, the nitrate + nitrite concentrations at zero oxygen concentrations did not change over the same interval. The silicate efflux from anoxic sediments is directly related to warming temperatures and is co-related to phosphate concentrations. The bottom water DSi:DIN molar ratios increased over three decades as DIN:DIP molar ratios decreased, suggesting strong nitrogen limitation compared to silicate and phosphate, and reveal significant plasticity in regeneration rates in the bottom waters that may be dependent on changes in the surface waters. Hypoxia and food web models based on a stationary equipoise of these amounts and ratios in surface and bottom waters will likely be deficient as coastal waters warm, acidification increases, and river water quality changes. Data refreshment and improved understanding of food web changes and warming futures are recommended.

The interconnections of some ocean currents in the Eastern Tropical Pacific are still a mystery to the scientific community. In the Mexican Pacific, the West Mexican Current (WMC) and the Costa Rica Coastal Current (CRCC) are two flows that are typically considered independent; however, some evidence of a subsurface connection when the Tehuantepec Bowl (TB) moves westward suggests otherwise. With the Hybrid Coordinated Ocean Model (HYCOM), we obtained a 19 years-run in a domain encompassing the Eastern Tropical Pacific Ocean. The numerical results were validated qualitatively and quantitatively by comparing them with remote and direct observations. We found a generally good agreement between the model results and those observations. To analyze the model results, we defined four transects perpendicularly to the coast, and surface and subsurface layers; in the long-term mean, the CRCC is disconnected from the WMC, in both layers. However, during the seasonal cycle, we found a close relationship between the general structures in thermocline topography and surface and subsurface large-scale circulation. The model results confirmed that the interconnection between the CRCC and the WMC depends on the dynamics of the TB, that on its seasonal migration moves westward, allowing the CRCC to travel along the coast further west than the Gulf of Tehuantepec (GT) and to connect with the WMC during spring at a subsurface level below the thermocline.

The seabed reflection amplitudes (SRAs) extracted from the sub-bottom profile have a strong correlation with the types and physical properties of the seabed sediments. In this paper, the SRAs distribution of classified seabed sediments is statistically obtained by calibration with seabed sampling results, discovering that SRAs on different seafloor sediment types exhibit Rayleigh distributions with varying parameters. Firstly, SRAs are compensated and enhanced, to improve their identification. Then, a novel classification method based on K–S test was proposed. This method measures the maximum distance between the cumulative distribution functions (CDF) of the unknown seabed and the calibrated sediment SRAs to check whether unknown samples belong to any of the known types. This proposed method only requires a small amount of seabed samples to automatically classify the seabed with high accuracy, and the model is simple, robust, and provides classification confidence.

Coastal upwellings significantly affect the thermohaline structure, gas exchange and the fluxes of nutrients in the upper layers. They are often characterized by complex spatial structure, which is related to impact of different factors – changes of bathymetry, impact of background currents, baroclinic instability and others. In this study, we use high-resolution NEMO numerical model-ling and satellite data to study in details several physical causes of the observed submesoscale features of the Black Sea coastal upwellings: . 1) the influence of capes on the separation of coastal upwelling from the coast and its further transformation into submesoscale eddies; 2) the influence of mesoscale eddies on the structure of coastal upwellings; 3) the role of river plumes and haline fronts in the transport of upwellings. Comparison of model and satellite data shows that the numerical calculations reasonably well reproduce the onset time, intensity and areas of upwellings. Further, we show that the discussed processes can significantly affect the intensity, evolution and propagation of coastal upwellings in the Black sea.