RESUMEN

Aquaculture is one of the fastest growing sectors in the world. However, this may come with a cost, as increasing aquatic production is likely to impose changes in the environment. To ensure ecosystem sustainability, it is essential to think on this larger scale. This study aims to use the Delft3D model suite to evaluate the ecological carrying capacity for bivalve production in the Sado Estuary (Portugal), under present and future conditions (2050). Scenarios for increased oyster production resulted in reductions of chlorophyll a associated with increased nutrient concentrations. In the most extreme production scenario, which considered an increase of 100 ha in production area, a predicted decrease of 90 % in phytoplankton biomass was observed. Climate change (CC) was incorporated as an increase in sea level and water temperature, as well as a reduction in river flow. Under present oyster production conditions, CC revealed contrasting patterns, i.e. an increase in chlorophyll a concentrations and a reduction in nutrients. These results suggest that CC has a positive effect in counteracting the impacts of increased oyster production, however further research is necessary. All scenarios point to reduced dissolved oxygen concentrations, highlighting the need to monitor this parameter. Given the difficulty in defining what are unacceptable impacts to the ecosystem it would be prudent to include a socio-ecological framework in the future, in order to integrate ecosystem services and the perception of local stakeholders.

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RESUMEN

The ecological importance of mixoplankton within marine protist communities is slowly being recognized. However, most aquatic ecosystem models do not include formulations to model a complete protist community consisting of phytoplankton, protozooplankton and mixoplankton. We introduce PROTIST, a new module for the aquatic ecosystem modelling software Delft3D-WAQ that can model a protist community consisting of two types of phytoplankton (diatoms and green algae), two types of mixoplankton (constitutive mixoplankton and non-constitutive mixoplankton) and protozooplankton. We employed PROTIST to further explore the hypothesis that the biogeochemical gradient of inorganic nutrient and suspended sediment concentrations drives the observed occurrence of constitutive mixoplankton in the Dutch Southern North Sea. To explore this hypothesis, we used 11 1D-vertical aquatic ecosystem models that mimic the abiotic conditions of 11 routine monitoring locations in the Dutch Southern North Sea. Our models result in plausible trophic compositions across the biogeochemical gradient as compared to in-situ data. A sensitivity analysis showed that the dissolved inorganic phosphate and silica concentrations drive the occurrence of constitutive mixoplankton in the Dutch Southern North Sea.

RESUMEN

Hypoxia is often thought of as the key factor responsible for fish kill events in coastal areas but fish kill events are too complex to be governed by a single factor. The events are influenced by a combination of chemical, biological and physical processes. Hydrodynamics play a key role in understanding the formation of hypoxia in shallow waters. This study aims to identify the settings of the physical forces that lead to a large-scale depletion of dissolved oxygen in Kuwait Bay at the northwest of the Arabian Gulf. The assessment, made with a validated three-dimensional numerical model (Alosairi and Alsulaiman, 2019), revealed that the pollution from the outfalls leads to nearfield depletion of dissolved oxygen but has only a minor effect on the bay-scale dissolved oxygen. This is a result of the strong dynamics of Kuwait Bay, which mixes the pollutant rapidly before it is transported seawards. Offshore, a low dynamic region has been identified near Jahra Bay which is susceptible to occasional dissolved oxygen depletion. Assessment of the physical forces revealed that the density-driven current and, to a greater extent the wind regime, controlled the formation of a hypoxic parcel near Jahra Bay. The combination of neap tides and low mixed winds reduced mixing and enabled the longer residence times of Kuwait Bay. These are the most critical circumstances, as the average dissolved oxygen can be reduced by 50% during summer. The circumstance resulting in low dynamics near Jahra Bay were also found to be effective in explaining algal blooms.

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