MINERALIZATION OF ORGANIC MATERIAL AND BACTERIAL DYNAMICS IN MISSISSIPPI RIVER PLUME WATER

Net remineralization rates of organic matter and bacterial growth rate s were observed in dark-bottle incubation experiments conducted in Jul y-August and February with water samples collected from sites in the M ississippi River plume of the Gulf of Mexico. Our objectives were to m easure site-specific degradation rates of labile dissolved and particu late organic matter, quantify the potential importance of bacteria in these processes, and examine the kinetics of degradation over time. Un filtered samples, and samples treated to remove (or dilute out) partic les larger than bacteria, were enclosed in 9-1 bottles and incubated i n, the dark for 3-5 d. Respiration rates and inorganic compound accumu lation rates were higher in summer than in winter and were highest in unfiltered surface samples at sites of intermediate salinities where p hytoplankton were most abundant. The ratio of ammonium accumulation to oxygen removal in summer experiments suggested that the mineralized o rganic material resembled ''Redfield''' stoichiometry. Chemical fluxes were greater in bottles containing large (>1-3 mu m) particles than i n the bottles with these particles removed, but bacterial activities w ere generally similar in both treatments. These results suggest that p article consumers were an important component of total organic matter degradation. However, these experiments may have underestimated natura l bacterial degradation rates because the absence of light could affec t the production of labile organic substrates by phytoplankton, In agr eement, with this hypothesis, bacterial growth rates tended to decreas e over time in summer in surface plume waters where phytoplankton were abundant. In conjunction with other data, our results indicate that h eterotrophic processes in the water column are spatially and temporall y dependent on phytoplankton production.