Form and function of the bulbus arteriosus in yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus) and blue marlin (Makaira nigricans): static properties
As part of an integrated study of the regulation of carbon fixation in the offshore Mississippi River plume, we measured the rates of N-15-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 muM with the highest concentrations primarily in the shoreward stations, while ammonium ranged from 0.17 to 0.44 muM, 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 (alpha-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.