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Aquatic Microbial Ecology

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AME 35:175-184 (2004)  -  doi:10.3354/ame035175

High rates of ammonium recycling drive phytoplankton productivity in the offshore Mississippi River plume

Boris Wawrik1, John H. Paul1,*, Deborah A. Bronk2, David John1, Mike Gray1

1University of South Florida, 140 7th Ave S, St. Petersburg, Florida 33701, USA
2The College of William and Mary, Virginia Institute of Marine Science, PO Box 1346, Gloucester Point, Virginia 23062, USA
*Corresponding author. Email:

ABSTRACT: As part of an integrated study of the regulation of carbon fixation in the offshore Mississippi River plume, we measured the rates of 15N-labeled ammonium and nitrate uptake in the surface plume waters from offshore to nearshore along the plume axis towards the Mississippi Delta. Concentrations of nitrate in the plume ranged from 0.19 to 2.5 μM with the highest concentrations primarily in the shoreward stations, while ammonium ranged from 0.17 to 0.44 μM, showing little spatial variability. Rates of ammonium uptake ranged from 16.5 to 260 nM h-1, and showed a strong trend of increasing values from offshore towards the Mississippi Delta. In contrast, nitrate uptake rates ranged from 3.2 to 25 nM h-1. The high rates of ammonium uptake in the presence of low ammonium concentrations and elevated nitrate was made possible by elevated rates of ammonium regeneration that exceeded ammonium uptake by 1.7 to 5.7-fold in the plume. The plume exhibited relatively low f-ratios and also contained elevated levels of Synechococcus as determined by flow cytometry and high levels of form IA (a-cyanobacterial) rbcL transcripts. These data suggest that a major portion of the carbon fixation observed in the offshore Mississippi River plume represents recycled production supported by high rates of ammonium regeneration.

KEY WORDS: Gulf of Mexico · Mississippi River plume · Nitrate uptake · Ammonium uptake · Nutrient cycling

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