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JPR Advance Access originally published online on June 24, 2004
Journal of Plankton Research 2004 26(11):1265-1275; doi:10.1093/plankt/fbh118
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Journal of Plankton Research Vol. 26 No. 11 © Oxford University Press 2004; all rights reserved

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

Frank J. Jochem*,{dagger}, Mark J. McCarthy and Wayne S. Gardner

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

* Corresponding Author: frank{at}jochem.net

Received February 19, 2004; accepted in principle April 8, 2004; accepted for publication June 15, 2004; published online June 24, 2004

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.


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