Concentrations and uptake of nitrate and ammonium in the Atlantic Ocean between 60 degrees N and 50 degrees S

The uptake rates and concentrations of nitrate and ammonium were determined during three research cruises in the Atlantic Ocean: At 20 W between 60 degrees N and 37 degrees N during June and July 1996, between the UK and Falkland Islands (48 degrees N-50 degrees S) during September and October 1997, and between South Africa and the UK (33 degrees S-48 degrees N) during May and June 1998. Euphotic zone concentrations of NO3- varied between 0.005 mu mol l(-1) in the northern and southern gyres to a maximum of 33 mu mol l(-1) within the Benguela upwelling system. At 70% of stations occupied in oligotrophic conditions (nominally defined as where NO3- < 0.05 mu mol l(-1)), surface elevations of NO3- were attributed to the photoinhibition of uptake mechanisms. NH4+ concentrations were much less variable (0.109 +/- 0.150 mu mol l(-1)) than NO3-, and showed the same general trend of minima within the gyres and maximum concentration (of 5.2 mu mol l(-1)) in the Benguela upwelling system. The microbial uptake of NO3- was significantly correlated to NO3- and chlorophyll concentrations, whilst NH4+ uptake was less dependent on NH4+ concentration and showed no association with other environmental variables. Variability in the nitrogen uptake: concentration ratio between oceanographic provinces was associated with physical conditions including the mixed-layer depth and the rate of diapycnal mixing; therefore, uptake rates cannot be predicted simply from the nutrient concentration. In contrast, the relationship between f-ratio (where the NO3- source is undefined, and may have both new and regenerated components) and NO3- concentration is defined robustly, so that the f-ratio can be predicted (within 95%) by either surface or depth integrated concentrations of NO3-. (c) 2006 Elsevier Ltd. All rights reserved.