Mitigation of Eutrophication and Hypoxia through Oyster Aquaculture: An Ecosystem Model Evaluation off the Pearl River Estuary.

Shellfish aquaculture has been proposed to abate eutrophication because it can remove nutrients via shellfish filter-feeding. Using a three-dimensional physical-biogeochemical model, we investigate how effective oyster aquaculture can alleviate eutrophication-driven hypoxia off the Pearl River Estuary. Results show that oysters reduce sediment oxygen consumption and thus hypoxia, by reducing both particulate organic matter directly and regenerated nutrients that support new production of organic matter. The hypoxia reduction is largest when oysters are farmed within the upper water of the low-oxygen zone, and the reduction increases with increasing oyster density although oyster growth becomes slower due to food limitation. When oysters are farmed upstream of the hypoxic zone, the farming-induced hypoxia reduction is small and it declines with increasing oyster density because the nutrients released from the farm can increase downstream organic matter production. An oyster farming area of 10 to 200 km yields a hypoxic volume reduction of 10% to 78%, equaling the impact of reducing 10% to 60% of river nutrient input. Our results demonstrate that oyster aquaculture can mitigate eutrophication and hypoxia, but its effectiveness depends on the farming location, areal size, and oyster density, and optimal designs must take into account the circulation and biogeochemical characteristics of the specific ecosystem.