The Effect of Temperature, Oxygen, Salinity, and Nutrient Enrichment on Estuarine Denitrification Rates Measured with a Modified Nitrogen Gas Flux Technique

Abstract

Nitrogen gas flux was measured from sediments taken from Narragansett Bay, Rhode Island, Boston Harbour, Massachusetts, and the Pawcatuck River Estuary, Rhode Island. In addition to studies of field cores taken directly from these systems, intact sediments were taken from Narragansett Bay and maintained in control and nutrient enriched mesocosms. Sediment denitrification was measured as a flux of N 2 gas from sediments in N 2-free chambers. The advantages of this technique are that it allows for the direct measurement of denitrification in undisturbed sediment cores under ambient conditions of dissolved nutrients, oxygen, and temperature. The challenge of this technique has been to correctly distinguish between N 2 fluxes produced by denitrification activity and fluxes of N 2 caused by sediment porewater de-gassing. In this study, anoxic 'control' cores were used to provide continuous checks on the magnitude of porewater de-gassing rates, and allowed measured rates of total N 2 flux to be corrected for this background flux. The use of anoxic control cores allowed measurements to begin soon after core collection, without the need for long pre-incubations. Observed rates of sediment denitrification ranged from 0 to 195 μmol N 2 m -2 h -1. Highest rates were found in the enriched mesocosms and at a sewage outfall site in Boston Harbour. Denitrification rates increased exponentially with temperature and were enhanced by added nutrients. Results from the anoxic control cores showed that even after 10 days, the background flux of N 2 from de-gassing porewaters (10-34 μmol N 2 m -2 h -1) remained a significant fraction of the overall N 2 flux. Although rates of sediment denitrification were stimulated by added nutrients, the overall percentage loss of N from these systems through denitrification did not keep pace with N enrichment, generally accounting for less than 20% of N inputs. Thus denitrification may not necessarily provide a significant pathway for alleviating eutrophication effects in coastal waters.