Latitudinal variability in phosphate uptake in the Central Atlantic

The role of P in regulating planktonic production in Atlantic waters was as sessed by the examination of the phosphate turnover time and uptake rate al ong a latitudinal transect across the Central Atlantic Ocean (27 degrees N to 36 degrees S). Phosphate uptake rates and the affinity for phosphate wer e higher for small (<0.8 mu m) organisms, compared with those >0.8 mu m. Ph osphate uptake rates were relatively low, resulting in long phosphate turno ver times (days), except in the surface waters south of 25 degrees S, which were also characterized by the highest uptake rates and affinity for phosp hate, and the smallest total P pools observed along the transect. The organ isms were found to realize their maximal phosphate uptake rates at ambient phosphate concentrations, suggesting the adequacy of the P supply to suppor t the requirements of organisms. These findings suggest that inorganic P wa s not limiting community production in most of the Central Atlantic, except for the area south of 25 degrees S, where P uptake could possibly be limit ed by P supply. The long turnover times generally observed in the Central A tlantic Ocean are in agreement with previous observations in oceanic system s elsewhere, suggesting that the observation that P is unlikely to be a lim iting resource for planktonic growth can be extrapolated to most of the ope n ocean. The combined rate of P excretion from planktonic organisms and the ir microbial grazers as dissolved organic phosphorus (DOP) represented 75% of the total phosphate uptake. This DOP does not reach a sufficient accumul ation as to drive an important downward flux of DOP, which represents a los s of only 9% of the P inputs into the biogenic layer. Hence, the high P upt ake rate of the planktonic community in the Central Atlantic provides P in excess to support primary production, leading to a release as DOP, which ap pears to be rapidly recycled in the biogenic layer, thereby maintaining an adequate P supply to fuel primary production.