Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000 |
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Authors: | Jochem Frank J; McCarthy Mark J; Gardner Wayne S |
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Institution: | The University of Texas at Austin, Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373, USA
Present Address: Florida International University, Marine Biology Program, 3000 NE 151 Street, North Miami, FL 33181, USA |
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Abstract: | 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 inspring 2000. Chlorophyll concentrations up to 30 µg L1were highest in the low- and mid-salinity regions (salinities8.528.2) and comparable to records of other years butextended over smaller areas than during periods of normal riverflow. Bacterial biomass (5.128.3 µg C L1)was at the low end of the range observed in normal flow years,decreased with distance from the river mouth and did not peakwith chlorophyll. Heterotrophic nanoflagellate abundance (1.44.0µg C L1) did not reflect phytoplankton and bacterialspatial distribution but peaked at 9.2 µg C L1at salinity 8.5. Microbial NH4+ regeneration rates were estimatedby 15NH4+ isotope dilution experiments for the whole microbialcommunity, under light and dark conditions, and for the <2µm bacterium-dominated size fraction. Microbial NH4+ regenerationrates (0.0180.124 µmol N L1 h1) werelow 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, suggestingthat bacterial mineralization was a significant component ofNH4+ recycling in some parts of the river plume. Higher NH4+regeneration in whole-water samples versus <2 µm fractionsprovided evidence for microbial grazing in regions where chlorophylland regeneration rates peaked and at two full-salinity stations. |
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