Semi-enclosed Coastal Ecosystem Research Articles

Thirty years of change: Assessing the dynamics of fish communities in Daya Bay, a semi-enclosed coastal ecosystem of the South China sea

Daya Bay is a semi-enclosed bay in the South China Sea which is a coastal ecosystem with significant ecological and economic value that is subjected to a variety of anthropogenic disturbances. However, long-term variation in fish community structure in this Bay remains uncertain. In this study, we aimed to explore the changing characteristics of fish species composition and community structure in Daya Bay during the past 30 years using data from 1989 to 2021 from bottom trawl surveys and historical literature data. As of now, a total of 544 species of fish have been identified in the Bay, belonging to 24 orders and 125 families. The structure of the fish community in the bay has changed over time. Firstly, there has been a clear decrease in the number of species and biomass of large fish. Secondly, the number of fish constituent orders and families has decreased. Moreover, both the average taxonomic distinctness and variation in taxonomic distinctness decreased from 1989 to 2021. Thirdly, there is low similarity among fishes at different time phases and distinct changes in the composition of dominant species. Present results indicate that the fish community structure in Daya Bay has tended to reflect a decline in diversity, shifts in composition, and shrinking of fish body size in the past 30 years. This study improves our understanding of fish species composition and its drivers over time in Daya Bay, providing important knowledge to support the protection and restoration of this ecosystem and the important ecosystem services it provides.

Variability of phytoplankton biomass and environmental drivers in a semi-enclosed coastal ecosystem (San Matías Gulf, Patagonian Continental Shelf, Argentina) using ocean color remote sensing (MODIS) and oceanographic field data: Implications for fishery resources

Annual cycles of phytoplankton biomass (using chlorophyll-a as a proxy) and seasonal patterns of environmental drivers were analyzed in a semi-enclosed coastal ecosystem (San Matías Gulf, SMG, Patagonian Continental Shelf, Argentina). Monthly climatologies (2003–2014) of satellite images of chlorophyll-a (Chla-sat), sea surface temperature (SST), photosynthetically available radiation (PAR), and euphotic depth (Z 1% ) were analyzed using Principal Component Analysis (PCA). Satellite environmental variables were evaluated together with oceanographic field data (temperature, nitrate, and chlorophyll-a). Thus, this study analyzes the Chla-sat data to describe the spatio-temporal distribution of the phytoplankton biomass in the SMG and investigates its relationship with the regional environmental drivers. Two contrasting phytoplankton annual cycles characteristic of these temperate waters were distinguished. PCA discriminated the inner and outer areas separated by a complex frontal system. The gulf's inner waters showed a bimodal phytoplankton cycle (maximums in autumn and spring) associated with the seasonal stratification of the water column in deeper areas and the consequent nitrate limitation in summer. The outer waters adjacent to the continental shelf showed unimodal cycles (maximum in autumn or spring) linked to the vertical mixing of the water column in shallow areas with moderate light intensity and nitrate availability. For both annual phytoplankton cycles, light is the main limiting factor in winter. Finally, the analyzed environmental patterns suggest that a tidal front favor feeding and spawning fishery resources in the north of the gulf. • Twelve years of chlorophyll-a MODIS data are analyzed in San Matías Gulf. • Environmental data from satellite and oceanographic cruises are investigated. • A frontal system separated two contrasting annual cycles of phytoplankton biomass. • Oceanographic features are favorable for feeding and spawning of fisheries resources.

Integration of Weather Research and Forecasting (WRF) model with regional coastal ecosystem model to simulate the hypoxic conditions

Regional ocean models require accurate weather data for atmospheric boundary conditions such as air temperature, wind speed, and direction to simulate the coastal environment. In this study, a numerical modelling framework was developed to simulate different physical, chemical, and biological processes in a semi-enclosed coastal ecosystem by integrating the Weather Research and Forecasting (WRF) model with a 3D hydrodynamic and ecosystem model (Ise Bay Simulator). The final analytic data of the global forecast system released by the National Centers for Environmental Prediction with a 0.25° horizontal resolution was used as an atmospheric boundary condition for the WRF model to dynamically downscale the weather information to a spatial and temporal fine resolution. This modelling framework proved to be an effective tool to simulate the physical and biogeochemical processes in a semi-enclosed coastal embayment. The WRF-driven ecosystem simulation and recorded Automated Meteorological Data Acquisition System (AMeDAS)-driven ecosystem simulation results were further compared with the observed data. The performance of both the recorded AMeDAS and WRF generated weather datasets were equally good, and more than 80% of the variation in bottom dissolved oxygen for shallow water and more than 90% for deep water was reproduced.

Nutrient distribution in a shallow subtropical lagoon, south Brazil, subjected to seasonal hypoxic and anoxic events

ABSTRACT The Conceição Lagoon, located in south Brazil, is a semi-enclosed coastal ecosystem that has seasonal hypoxic and anoxic conditions in its vertically stratified central region, characterized as a site of retention and mineralization of organic matter. This study investigates water column dynamics in the central region of the Conceição Lagoon (CCL) and its relation to physical and chemical variables, in order to understand the hypoxic and anoxic events. Surface, halocline and bottom waters were evaluated at three sampling sites along the CCL. The samples were collected in triplicate during the summer, fall and winter of 2014. Hypoxic and/or anoxic events occurred in the summer (1/21) at the halocline (3 m) and bottom (4 m) waters, and in the fall (2/5) in the bottom water (4.5 m). Positive values of apparent oxygen utilization showed mineralization processes in the halocline and bottom waters. The lowest vertical stratification index was recorded in August (southern winter), which was associated with wind speed (14.7 m.s-1) and direction (southern quadrant). Nutrient concentrations were higher in winter, related to increasing of water mixing. This was the first study to evaluate the dynamics of hypoxic and anoxic events in the CCL and how nutrients respond to the physical structure of the water column.

Heterogeneous distribution in sediments and dispersal in waters of Alexandrium minutum in a semi-enclosed coastal ecosystem

Within the framework of research aimed at using genetic methods to evaluate harmful species distribution and their impact on coastal ecosystems, a portion of the ITS1rDNA of Alexandrium minutum was amplified by real-time PCR from DNA extracts of superficial (1–3cm) sediments of 30 subtidal and intertidal stations of the Bay of Brest (Brittany, France), during the winters of 2013 and 2015. Cell germinations and rDNA amplifications of A. minutum were obtained for sediments of all sampled stations, demonstrating that the whole bay is currently contaminated by this toxic species. Coherent estimations of ITS1rDNA copy numbers were obtained for the two sampling cruises, supporting the hypothesis of regular accumulation of A. minutum resting stages in the south-eastern, more confined embayments of the study area, where fine-muddy sediments are also more abundant. Higher ITS1rDNA copy numbers were detected in sediments of areas where blooms have been seasonally detected since 2012. This result suggests that specific genetic material estimations in superficial sediments of the bay may be a proxy of the cyst banks of A. minutum. The simulation of particle trajectory analyses by a Lagrangian physical model showed that blooms occurring in the south-eastern part of the bay are disconnected from those of the north-eastern zone. The heterogeneous distribution of A. minutum inferred from both water and sediment suggests the existence of potential barriers for the dispersal of this species in the Bay of Brest and encourages finer analyses at the population level for this species within semi-enclosed coastal ecosystems.

Seasonal variation and sources of dissolved trace metals in Maó Harbour, Minorca Island

The environmental conditions of semi-enclosed coastal water-bodies are directly related to the catchment, human activities, and oceanographic setting in which they are located. As a result of low tidal forcing, and generally weak currents, waters in Mediterranean harbours are poorly renewed, leading to quality deterioration. Here, we characterise the seasonal variation of trace metals (i.e. Co, Cd, Cu, Fe, Mo, Ni, Pb, and Zn) in surface waters, and trace metal content in sediments from Maó Harbour, a semi-enclosed coastal ecosystem in the NW Mediterranean Sea. Our results show that most of the dissolved trace metals in the waters of Maó Harbour exhibit a marked inner-outer concentration gradient, suggesting a permanent input into the inner part of the harbour. In general, metal concentrations in the waters of Maó Harbour are higher than those in offshore waters. Concentration of Cu (21±8nM), Fe (9.2±3.2nM) and Pb (1.3±0.4nM) are particularly high when compared with other coastal areas of the Mediterranean Sea. The concentration of some metals such as Cu and Zn increases during summertime, when the human population and boat traffic increase during the tourism season, and when resuspension from the metal enriched sediments is higher. The evaluation of the metal sources in the harbour reveals that, compared with other putative sources such as runoff, aerosol deposition and fresh groundwater discharges, contaminated sediments are the main source of the metals found in the water column, most likely through vessel-driven resuspension events. This study contributes to the understanding of the processes that control the occurrence and distribution of trace metals in Maó Harbour, thus aiding in the effective management of the harbour, and enhancing the overall quality of the seawater ecosystem.

Trophic resource use by macrozoobenthic primary consumers within a semi-enclosed coastal ecosystem: Stable isotope and fatty acid assessment

The diet of different macrozoobenthic trophic groups was investigated in the Arcachon Bay—a semi-enclosed macrotidal ecosystem that shelters the largest Zostera noltei seagrass meadow in Europe—in early spring and late summer 2009, using stable isotopes and fatty acids. Fatty acid profiles and literature information about the biology and physiology of benthic consumers were combined to identify the main organic matter sources for the benthic primary consumers. An isotope mixing model was then run to evaluate the contribution of each organic matter source to each identified trophic group (suspension feeders, sub-surface deposit feeders, micro-and macrograzers, suspension-oriented interface feeders and deposit-oriented interface feeders). Variations in organism' diets with respect to both habitats (intertidal seagrass meadows, intertidal bare sediments and subtidal bare sediments) and study periods were also investigated. At the scale of this study, it appeared that the diet of macrozoobenthos primary consumers was based exclusively on autochthonous material (no use of terrestrial organic matter): mainly microphytobenthos, seagrasses and their epiphytes, and phytoplankton. In addition, the different trophic groups relied on different organic matter pools: for instance, suspension feeders mainly fed on microphytobenthos and phytoplankton, whereas subsurface deposit feeders fed on microphytobenthos, decayed seagrasses and bacteria, and grazers mainly fed on microphytobenthos, and seagrasses and their epiphytes. The same pattern was observed in both early spring and late summer, indicating a stability of the benthic system at a six-month time scale. Finally our results showed that, in Arcachon Bay, the seagrass meadow directly or indirectly (through detritus) plays a significant role in the diet of most benthic consumers.

Hydrodynamic prevention of eutrophication in the Bay of Brest (France), a modelling approach

The Bay of Brest is a semi-enclosed coastal ecosystem where primary production is nutrient-limited, even if huge nutrients loading from tributaries are present. The most striking feature of the bay is the semi-diurnal tidal influence, resulting in large water exchange with the continental shelf. A historical study of the available data has shown the steadiness of this ecosystem during the two last decades in spite of increasing eutrophic conditions. This study has focused on hydrodynamic exchange which is one of the factors supposed to explain the resistance of this ecosystem to eutrophication: this stirring hinders the formation of a persistent upper mixed layer where phytoplankton would be in contact with nutrient-rich brackish waters and available light. Moreover, horizontal tidal currents lead to huge exchanges with the Iroise Sea and, then, to big losses of nutrients and living matter. To study this hydrodynamic influence thoroughly, a physical/biological model of this bay has been developed. This box model, based on the horizontal tidal circulation, has been developed thanks to “ELISE”, an ecological modelling software and, then, tuned and validated on two data sets corresponding with the years 1977 and 1993. The model has allowed us to quantify the influence of hydrodynamics, climatic conditions and biological factors on biogeochemical processes in this ecosystem. It contributes to explain the good resistance of the Bay of Brest ecosystem to eutrophic conditions; both the hydrodynamic properties of this bay and the grazing pressure have prevented it from disturbances caused by high nitrogen loading from the watersheds and explain the steadiness of phytoplankton stocks in spite of increased loading. So, these results allow us to say that, even if nitrogen inputs increase continues, phytoplankton stocks will not increase in significant proportions. Nevertheless, changes in the phytoplanktonic populations may occur if such an enrichment continues.

Resistance of a coastal ecosystem to increasing eutrophic conditions: the Bay of Brest (France), a semi-enclosed zone of Western Europe

The Bay of Brest is a semi-enclosed coastal ecosystem receiving high nutrients loading from freshwater inputs. In order to analyse the response of phytoplankton stocks to increasing eutrophic conditions, a survey of the annual cycle of hydrographic properties, nutrients and chlorophyll a concentrations, and carbon uptake rates was performed at four stations in 1993. This database has been compared to earlier measurements performed during several comparable surveys within the last 20 years. As compared to the seventies, a doubled nitrate loading is now entering this ecosystem, which is related to increased agricultural activities on the drainage basins, while the geographical origin of the nitrate input has been modified. As a result of these anthropogenic modifications, summer averaged Si/N stoichiometric balance has decreased during the two last decades but, contrary to what has been observed in other coastal ecosystems, phytoplankton stocks have not increased. Several ecological factors have hindered eutrophication: the high hydrodynamic mixing with adjacent marine waters, caused by the macrotidal regime, induces important nutrients losses, temperature and mostly light limit primary production while Si and P high recycling maintain nitrogen limitation in this ecosystem. Conjunction of these non-anthropogenic factors explains the global stability of phytoplankton stocks.

Phytoplankton dynamics in relation to the biogeochemical cycle of silicon in a coastal ecosystem of western Europe

The Bay of Brest, France, a typical semi-enclosed coastal ecosystem (159 km2) of western Europe, was studied during spring 1992 with respect to the biogeochemical cycle of silicon. Three periods of nutrient and phytoplankton dynamics (chlorophyll a, biogenic silica, species composition), were distinguished during spring 1992, each corresponding, respectively, to a bloom of Thalassiosira sp. and Skeletonema costaturn during early spring (April], Rhizosolenia sp. during mid-spring (May) and Chaetoceros sociale during late spring (June). During each period the production of biogenic silica (the mean rate of spring biogenic silica production was 13 mm01 Si m-' dl ) , derived from I4C primary production measurements, size fractionation experiments and appropriate Si: C ratios, has been compared with the sum of the silicic acid inputs to the bay originating from rivers, from the adjacent Iroise Sea and from the sediments. From this comparison, it is concluded that (1) the early spring diatom bloom was mainly sustained by silicic acid from the watershed, (2) recycling of silicic acid within the water column played a major role during mid-spring to sustain the bloom of Rhizosolenia sp. and (3) silicic acid recycling at the sediment-water interface was the main contributor to the silica production during the late spring bloom. On a seasonal basis, the riverine inputs of Si (net source) balance the Si burial in sediments (net sink), and the contribution of the sediment to the silica production equals that of the watershed. The factors that govern the quantitative and qualitative variations of phytoplankton blooms during these periods are discussed. In these nitrate-rich coastal waters, support is given to the hypothesis of Si-limitation of the diatom growth, at least during the early spring period when inputs of silicic acid from the watershed represented the major contribution to the silica production.