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JPR Advance Access published online on June 24, 2004
Journal of Plankton Research, doi:10.1093/plankt/fbh118
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Received February 19, 2004
Accepted June 15, 2004

Article

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

Frank J. Jochem 1*, Mark J. McCarthy 2, Wayne S. Gardner 2

1 Present address: Florida International University, Marine Biology Program, 3000 NE 151 Street, North Miami, FL 33181, USA
2 The University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373, USA

* To whom correspondence should be addressed. E-mail: frank{at}jochem.net.


  Abstract

Microbial potential-uptake and regeneration rates of ammonium (NH4+) were studied along a salinity gradient (salinity 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. Bacteria 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 bacteria 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 bacteria-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 4 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.


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