RATES OF NITRIFICATION ALONG AN ESTUARINE GRADIENT IN NARRAGANSETT BAY

Rates of pelagic nitrification, measured using N-Serve-sensitive [C-14 ]bicarbonate uptake, varied by as much as an order-of-magnitude among three sites along the salinity gradient of Narragansett Bay (Rhode Isl and, United States). Rates were always higher at the Providence River estuary site (0.04-11.2 mumol N l-1 d-1) than at either the lower Narr agansett Bay site (0.02-0.98 mumol N l-1 d-1) or the freshwater Blacks tone River site (0.04-1.7 mumol N l-1 d-1). Although temperature was t he most important variable regulating the annual cycle of nitrificatio n, ammonium concentrations were most likely responsible for the large differences in rates among the three sites in summer. At the levels fo und in this estuarine system, salinity and concentrations of oxygen or total suspended matter did not appear to have a direct measurable eff ect on nitrification and pH did only occasionally. Nitrification playe d an important role in the nitrogen cycle at all three sites. In Narra gansett Bay, nitrification contributed 55% of the NO2- and NO3- enteri ng annually, and was the major source during spring and summer. Water from offshore was the only other large source of NO2- and NO3-, contri buting 34%. High summer rates of nitrification could support much of t he phytoplankton uptake of NO2- and NO3-. In the Providence River estu ary, the largest annual input of NO2- and NO3- was from rivers (54%), although nitrification (28%) and water from lower portions of the bay (11%) also made large contributions. Again, nitrification was most imp ortant in the summer. The high rates of nitrification in the Providenc e River estuary during summer were also likely to be important in term s of oxygen demand, and the production of nitric and nitrous oxides. I n the Blackstone River, NO2- and NO3- concentrations increased as the river flowed through Rhode Island, and nitrification was a possible so urce.