U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameMicrobial ammonium recycling in the Mississippi River plume during the drought spring of 2000.
Document AuthorJochem, F. J.
McCarthy, M. J.
Gardner, W.S.
Short DescriptionJ Plankton Res 26:1265-1275
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2004

Abstract: Microbial potential uptake and regeneration rates of ammonium (NH4+) 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 mug 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 mug 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 mug C L-1) did not reflect phytoplankton and bacterial spatial distribution but peaked at 9.2 mug C L-1 at salinity 8.5. Microbial NH4+ regeneration rates were estimated by (NH4+)-N-15 isotope dilution experiments for the whole microbial community, under light and dark conditions, and for the <2 mum bacterium-dominated size fraction. Microbial NH4+ regeneration rates (0.018-0.124 mumol N L-1 h(-1)) were low relative to previous reports and peaked at salinity 28. Total NH4+ regeneration rates were higher than those in the <2 mum size fraction at only four stations, suggesting that bacterial mineralization was a significant component of NH4+ recycling in some parts of the river plume. Higher NH4+ regeneration in whole-water samples versus <2 mum fractions provided evidence for microbial grazing in regions where chlorophyll and regeneration rates peaked and at two full-salinity stations.