Relative Contributions of DNRA and Denitrification to Nitrate Reduction in Thalassia testudinum Seagrass Beds in Coastal Florida (USA)

Seagrass beds are vulnerable to eutrophication (nutrient loading) and declining worldwide. To quantify the fate of nitrogen (N) inputs, intact sediment cores were incubated in a continuous-flow system with N-15 enrichments to compare N consumption and efflux pathways within and near seagrass (Thalassia testudinum) beds in St. Joseph Bay, Florida. Sediment oxygen demand and total ammonium (NH4+) efflux were greater (p<0.001) in vegetated versus unvegetated sediments, suggesting that seagrasses enhance organic matter remineralization. Denitrification rates were 2-20x greater than estimates of potential dissimilatory nitrate reduction to ammonium (DNRA). The effect of vegetation on denitrification rates was inconsistent. Direct denitrification of overlying water nitrate and N fixation rates were low, suggesting tight coupling between remineralization, nitrification, and denitrification. DNRA rates were lower than denitrification and consistently greater in vegetated sediments. DNRA rate measurements are conservative if sediment cation exchange decreases the fraction of (NH4+)-N-15 reaching overlying water, especially in vegetated sediments, where rates of nitrate-induced ammonium fluxes were observed. Coupled nitrification-denitrification was the major N loss pathway in this system, as evidenced by the lack of N-15-labeled N-2 production in isotope-enriched cores. Using measured sediment oxygen demand and NH4+ fluxes as an indicator of organic matter quality and quantity, these results are consistent with previous work showing that labile organic matter helps regulate the balance between N removal and internal recycling pathways in seagrass systems, which has implications for coastal management strategies to address eutrophication.