Concentrations and uptake of nitrate and ammonium in the Atlantic Ocean between 60∘N60∘N and 50∘S50∘S

The uptake rates and concentrations of nitrate and ammonium were determined during three research cruises in the Atlantic Ocean: At 20∘W20∘W between 60∘N60∘N and 37∘N37∘N during June and July 1996, between the UK and Falkland Islands (48∘N–50∘S)(48∘N–50∘S) during September and October 1997, and between South Africa and the UK (33∘S–48∘N)(33∘S–48∘N) during May and June 1998. Euphotic zone concentrations of NO3- varied between 0.005μmoll-1 in the northern and southern gyres to a maximum of 33μmoll-1 within the Benguela upwelling system. At 70% of stations occupied in oligotrophic conditions (nominally defined as where NO3-<0.05μmoll-1), surface elevations of NO3- were attributed to the photoinhibition of uptake mechanisms. NH4+ concentrations were much less variable (0.109±0.150μmoll-1) than NO3-, and showed the same general trend of minima within the gyres and maximum concentration (of 5.2μmoll-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-.