Community Metabolism and Nutrient Cycling in the Mississippi River Plume: Evidence for Intense Nitrification at Intermediate Salinities

Community respiration, net nutrient fluxes and heterotrophic bacterial production were investigated in the Mississippi River (USA) plume during May 1992 using dark bottle incubations of unfiltered water. Highest rates of community O2 consumption and dissolved inorganic carbon regeneration were observed at intermediate (10 to 27%0) plume salinities. Plume surface 02 consumption rates were 2- to 4-fold greater than rates reported previously during the summer and winter. Heterotrophic bacterial production ([3H]-leucine incorporation) was also highest at intermediate salinities and 2- to 4-fold greater than rates reported from other seasons. Net regeneration of NH4+ was observed in the 0 to l8%0 region of the plume while low rates of net NH4+ consumption were observed at 27%0. Net NO2- regeneration in the Mississippi River suggested the occurrence of nitrification m the fresh waters of the delta. Serendipitous observations of rapid NO3- regeneration at 18 and 27%0 indicated the development of intense nitrification at intermediate plume salinities. Nitrification accounted for 20 to \u3e50 % of the community 02 demand at 18 and 27%0. These data indicated that nitrification was an important component of the plume nitrogen cycle and contributed significantly to oxygen consumption in the plume