Impacts of New England Oyster Aquaculture on Sediment Nitrogen Cycling: Implications for Nitrogen Removal and Retention

In this three-year, seasonal study of sediment nitrogen (N) biogeochemistry, we used stable isotope techniques to measure the N cycling reactions nitrification, denitrification (DNF), and dissimilatory nitrate reduction to ammonium (DNRA). Rates were compared to dissolved inorganic nitrogen exchanges between the sediment and overlying water to assess denitrification efficiency (DE). All measurements were performed on sediments underlying (oyster) and removed from (control) oyster aquaculture in two oyster farms in the Northeast, USA. Overall, DNF was small relative to release of DIN from sediment to the overlying water. Based on DE, both sites were found to favor N recycling over N removal, releasing 81% to 93% more N back to the overlying water than was removed via DNF. However, DE was enhanced by 50 to 100% in the oyster cores at both sites. Collectively, these data indicate that presence of oysters enhances DNF in warmer seasons and favors DNF over DNRA, but this enhancement of N removal via DNF is modest relative to the amount of DIN released to the overlying water at these sites. Results also show that oysters affect the balance of some, but not all, N cycle processes simultaneously. As a result, DNF should be concurrently measured with other N cycle processes in order to more accurately assess changes to the sediment N-cycle resulting from oyster biodeposition