AMMONIUM RECYCLING VERSUS DENITRIFICATION IN CHESAPEAKE BAY SEDIMENTS

Contemporaneous measurements are reported for nitrification, denitrification, and net sediment-water fluxes of NH4+ and NO3- in the mesohaline region of Chesapeake Bay. Seasonal cycles over a 2-yr period were characterized by a midsummer maximum in NH4+ efflux to the overlying water and a May peak in NO3- removal from water by sediments. Coherent temporal patterns for nitrification and denitrification were observed, with relatively high values in spring and fall and virtual elimination of both processes in summer. Indirect measurements indicate that nitrification was limited by the shallow O2 penetration (< 1 mm) here compared to reports for other marine sediments (2-6 mm). In addition, a strong positive correlation between the two processes suggested that denitrification was generally controlled by nitrification. Comparisons of NO3- fluxes and net nitrification rates (nitrification minus NO3- reduction to NH4+) revealed that measurements of dentrification with the acetylene block method systematically underestimated actual rates. Rates of N2 loss in denitrification were similar to NH4+ recycling fluxes to the overlying water in spring and fall, but in summer negligible denitrification contributed to enhanced NH4+ recycling. These results suggest that inhibition of denitrification in eutrophic estuaries such as Chesapeake Bay may reinforce the effects of nutrient enrichment by allowing increased rates of NH4+ recycling.