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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 C L −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 nanoflagellate abundance (1.4–4.0 μg C L −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 &lt;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 &lt;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 NH 4  +  regeneration in whole-water samples versus &lt;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