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Microbial Ammonium Cycling in the Mississippi River Plume, Gulf of Mexico
Microbial ammonium cycling in the Mississippi River plume during the drought spring
of 2000
Frank J. Jochem, Mark J. McCarthy, Wayne S. Gardner
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 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 NH4+
regeneration rates were estimated by 15NH4+ isotope dilution experiments for the whole microbial
community, under light and dark conditions, and for the <2 µm bacterium-dominated size fraction.
Microbial NH4+ regeneration rates (0.018–0.124 µmol 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 µm
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 µm fractions provided evidence for microbial grazing in regions where chlorophyll and
regeneration rates peaked and at two full-salinity stations.
Published in: Journal of Plankton Research 26: 1265-1275; 2004
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