Sediment-Water Nitrogen Exchange along the Potomac River Estuarine Salinity Gradient

Observations of N-2 efflux in estuarine sediments across the salinity gradient of the tidal Potomac River, a eutrophic subestuary of the Chesapeake Bay, were used to evaluate environmental controls of microbial denitrification. Rates of denitrification were measured using N-2:Ar ratios in core incubations and were similar to other nitrogen-enriched estuaries, with summer and spring N-2-N efflux rates averaging 54 +/- 47 and 153 +/- 97 mu mol m(-2) h(-1), respectively. The paradigm of higher denitrification rates at lower salinities was not supported by observations during summer and spring conditions along this estuarine salinity gradient. Low bottom water oxygen concentrations in the lower, more saline part of the estuary resulted in low rates of coupled nitrification/denitrification. The most favorable region for denitrification in the tidal Potomac River occurred where changes in salinity were most rapid and oxygen concentrations were not depleted, with high rates observed within the estuarine turbidity maximum (ETM) zone. Overall, the key role of salinity in the tidal Potomac River in controlling denitrification appears to be in the focusing of materials into ETM and providing stratification in the lower estuary that restricts the vertical exchange of oxygen necessary for coupled nitrification/denitrification.