Oyster Aquaculture Impacts on Sediment Nitrogen Cycling and Efficacy as a Nutrient Bioextraction Tool in a Tributary of Chesapeake Bay

Oysters are popularly thought to improve water quality by filtering plankton and associated nutrients from the water column, and oyster aquaculture and reef restoration have thus been proposed as tools to reduce nutrient pollution in coastal areas. However, oysters’ net effect on water quality, including indirect impacts associated with altered sediment nutrient cycling, remains poorly understood. This study assessed the impacts of oyster aquaculture on sediment nitrogen (N) processes—including denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and fluxes of dissolved nutrients—at a commercial oyster farm in Cherrystone Inlet, VA, a tributary of Chesapeake Bay. Denitrification was significantly enhanced in farm sediments, but rates were low (< 20 µm N m-2 h-1) and appeared to be limited by sediment anoxia and inhibited nitrification. Furthermore, DNRA was the dominant nitrate reduction pathway, accounting for an average of 70% of nitrate reduction. Ammonium flux to the water column was the most significant measured pathway in the farm, exceeding 900 µm N m-2 h-1 in summer. Extrapolating the observed annual rates to the area leased for aquaculture in the inlet, N extractive processes—including harvest, enhanced denitrification, and enhanced accumulation in sediment—could remove 160% of the annual N load. However, the enhanced ammonium flux from sediment to the water column was comparable in magnitude to total N extraction, potentially supporting local N recycling and eutrophication. However, negative impacts associated with eutrophication were not currently apparent in the inlet, likely due to active farm management and high tidal flushing. Thus, oyster aquaculture may be an effective tool to reduce nutrient concentrations in this location and similar water bodies with moderate N loads, high tidal flushing, and significant area for aquaculture expansion.