1534 / 2024-09-27 14:08:09

Oyster habitats enhance denitrification in a heavily degraded and polluted marine system

Oyster Reefs,ecosystem restoration,denitrification,ecosystem services,eutrophication

Oyster reefs are one of the most degraded marine habitats, with at least 85% lost globally, likely an underestimate in the highly developed coastlines of Asia. Oyster reefs in Hong Kong, for example, have been exploited for millennia and are now likely functionally extinct. In addition, Hong Kong’s western waters suffer from extremely high levels of nitrogen inputs from the Pearl River Estuary, which houses a population of over 80 million people. Yet, even in heavily degraded states oyster habitats provide multiple ecosystem services. One of the most underappreciated of these services is reduction of coastal pollution by filtering organic matter from the water column and by facilitating nitrogen removal through denitrification by their associated microbial communities. Therefore, to understand the current and potential future capacity of denitrification by oyster reefs in Hong Kong, we quantified filtration rates of oysters and denitrification by oyster and adjacent soft-sediment habitats at multiple locations across different seasons. Filtration rates of native Hong Kong oyster species are among the highest rates recorded globally, especially at summer temperatures (up to 30 l^-1.hr^-1 per indiv.). We found that mixed-species oyster habitat (primarily Magallana spp.) had N-N₂ flux of ~1500 mmol m^-2 hr^-1, over 8 times greater than the bare soft-sediment. While the oxygen demand increased with oyster density, a potential issue in seasonally hypoxic environments, the nitrogen remineralization efficiency was similar to rates seen in other regions. Importantly, denitrification rates are comparable to those found in well studied regions globally (e.g., Chesapeake Bay, USA). Therefore, while oyster reefs in Hong Kong have been exploited to near functional extinction, our data show that large-scale restoration would substantially benefit the quality of coastal waters through the high filtration and denitrification capacity of the native oysters in the region.