OYSTERS

HOW DO OYSTERS REMOVE NITROGEN?

Nitrogen is natural and necessary, yet nitrogen levels above natural levels can cause algal blooms and eutrophication of coastal systems. Bivalve shellfish, including oysters (Crassostrea virginica), can help mitigate the effects of too much nitrogen through filtration, accumulation of nitrogen in their shells and tissues, and enhancement of sediment denitrification. Denitrification is a natural process that converts nitrogen which can be used by algae, like nitrate (NO3–), to harmless di-nitrogen gas (N2). Strategies to remove nitrogen and improve degraded water quality are multi-faceted, and impacts vary depending on the system’s location, hydrology, and nitrogen load. Restoration and aquaculture of bivalve shellfish, like oysters, represent one potential tool for reducing nitrogen and improving water quality in coastal systems. The purpose of this document is to describe how oysters and oyster reefs remove nitrogen. Because oysters build reef structure, which traps organic matter and serves as habitat for other organisms, they may have a different effect on nitrogen removal than other shellfish. The document is intended for policymakers, environmental organizations, and coastal residents who want to know about oysters’ role in improving water quality through nitrogen removal. Click here to read more about this work.

OYSTER AQUACULTURE AND OYSTER REEFS HAVE COMPARABLE DENITRIFICATION RATES

Oysters have decreased globally from overharvesting and disease, leading to a loss of associated ecosystem services such as improved water quality through filtration and nitrogen (N) removal. At the same time, an increase in N loading from watersheds has led to eutrophication and impaired coastal waterbodies. Oyster aquaculture and reef restoration may help remove N by enhancing sediment denitrification, the microbial conversion of bioavailable N to inert N2 gas. Yet, it is unknown if oyster aquaculture and oyster reefs have different effects on sediment N cycling. Sediment fluxes of N2 gas, nitrate + nitrite, ammonium, and sediment oxygen demand were measured from an oyster aquaculture farm, an oyster sanctuary reef closed for harvest, and a harvestable oyster reef in the Rappahannock River, Virginia (USA). Fluxes from oyster sites were compared to bare reference sites without oysters. We hypothesized that oyster sites would have higher denitrification rates (net positive N2 fluxes) compared to reference sites and that denitrification enhancement would differ between aquaculture and reefs. Both oyster aquaculture and reefs increased sediment denitrification compared to reference sites, and the magnitude of this increase did not differ by oyster growing condition, despite differences in hydrology and depth. This study demonstrates that oyster aquaculture and reefs have equivalent effects on sediment N cycling and provide N removal benefits via enhanced denitrification. This research is part of my master’s thesis at the University of Florida and was presented at the National Shellfisheries Association Annual Meeting (virtual) and the Association for the Sciences of Limnology and Oceanography Aquatic Sciences Meeting (virtual) in 2021.