Net sediment N2 fluxes in a southern New England estuary: variations in space and time

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N 2 across the sediment–water interface over an annual cycle using the N 2 /Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N 2 –N m −2 h −1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (−5 to −650 μmol N 2 –N m −2 h −1 ) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.