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Microbial potential uptake and regeneration rates of ammonium (NH 4 + ) were studied along a salinity gradient (salinities 0.2-34.4) in the Mississippi River plume during an extreme drought in spring 2000. Chlorophyll concentrations up to 30 μg L -1 were highest in the low- and mid-salinity regions (salinities 8.5-28.2) and comparable to records of other years but extended over smaller areas than during periods of normal river flow. Bacterial biomass (5.1-28.3 μg CL -1 ) was at the low end of the range observed in normal flow years, decreased with distance from the river mouth and did not peak with chlorophyll. Heterotrophic mncflagellate abundance (1.4-4.0 μg CL -1 ) did not reflect phytoplankton and bacterial spatial distribution but peaked at 9.2 μg C L -1 at salinity 8.5. Microbial NH 4 + regeneration rates were estimated by 15 NH 4 + isotope dilution experiments for the whole microbial community, under light and dark conditions, and for the <2 μm bacterium-dominated size fraction. Microbial. NH 4 + regeneration rates (0.018-0.124 μmol N L -1 h -1 ) were low relative to previous reports and peaked at salinity 28. Total NH 4 + regeneration rates were higher than those in the <2 μm size fraction at only four stations, suggesting that bacterial mineralization was a significant component of NH 4 + recycling in some parts of the river plume. Higher AH 4 + regeneration in whole-water samples versus <2 μm fractions provided evidence for microbial grazing in regions where chlorophyll and regeneration rates peaked and at two full-salinity stations.

Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000