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

Abstract: The uptake rates and concentrations of nitrate and ammonium were determined during three research cruises in the Atlantic Ocean: At between and during June and July 1996, between the UK and Falkland Islands during September and October 1997, and between South Africa and the UK during May and June 1998. Euphotic zone concentrations of varied between in the northern and southern gyres to a maximum of within the Benguela upwelling system. At 70% of stations occupied in oligotrophic conditions (nominally defined as where ), surface elevations of were attributed to the photoinhibition of uptake mechanisms. concentrations were much less variable than , and showed the same general trend of minima within the gyres and maximum concentration (of ) in the Benguela upwelling system. The microbial uptake of was significantly correlated to and chlorophyll concentrations, whilst uptake was less dependent on 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 source is undefined, and may have both new and regenerated components) and concentration is defined robustly, so that the f-ratio can be predicted (within 95%) by either surface or depth integrated concentrations of . [Copyright &y& Elsevier]