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Marine Biology Program
Florida International University
3000 NE 151 Street
North Miami, FL 33181; USA
Phone: (305) 919 5882
Fax: (305) 919 5896

Office: AC-I 379
Lab: AC-II 350
Lab phone: (305) 919 4226

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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 0.234.4) in the Mississippi River plume during an extreme drought in spring 2000. Chlorophyll concentrations up to 30 g L-1 were highest in the low- and mid-salinity regions (salinities 8.528.2) and comparable to records of other years but extended over smaller areas than during periods of normal river flow. Bacterial biomass (5.128.3 g 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 (1.44.0 g C L-1) did not reflect phytoplankton and bacterial spatial distribution but peaked at 9.2 g 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 m bacterium-dominated size fraction. Microbial NH4+ regeneration rates (0.0180.124 mol 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 m 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 m 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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