Concentrations and uptake of nitrate and ammonium in the Atlantic Ocean between and

The uptake rates and concentrations of nitrate and ammonium were determined during three research cruises in the Atlantic Ocean: At 20 ∘ W between 60 ∘ N and 37 ∘ N during June and July 1996, between the UK and Falkland Islands ( 48 ∘ N – 50 ∘ S ) during September and October 1997, and between South Africa and the UK ( 33 ∘ S – 48 ∘ N ) during May and June 1998. Euphotic zone concentrations of NO 3 - varied between 0.005 μ mol l - 1 in the northern and southern gyres to a maximum of 33 μ mol l - 1 within the Benguela upwelling system. At 70% of stations occupied in oligotrophic conditions (nominally defined as where NO 3 - < 0.05 μ mol l - 1 ), surface elevations of NO 3 - were attributed to the photoinhibition of uptake mechanisms. NH 4 + concentrations were much less variable ( 0.109 ± 0.150 μ mol l - 1 ) than NO 3 - , and showed the same general trend of minima within the gyres and maximum concentration (of 5.2 μ mol l - 1 ) in the Benguela upwelling system. The microbial uptake of NO 3 - was significantly correlated to NO 3 - and chlorophyll concentrations, whilst NH 4 + uptake was less dependent on NH 4 + 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 NO 3 - source is undefined, and may have both new and regenerated components) and NO 3 - concentration is defined robustly, so that the f-ratio can be predicted (within 95%) by either surface or depth integrated concentrations of NO 3 - .