Net sediment N 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 across the sediment-water interface over an annual cycle using the N/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-N m h 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-N m h) 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. [ABSTRACT FROM AUTHOR]