Latitudinal variability in phosphate uptake in the Central Atlantic

The role of P in regulating planktonic production in Atlantic waters was assessed by the examination of the phosphate turnover time and uptake rate along a latitudinal transect across the Central Atlantic Ocean (27 degrees N to 36 degrees S). Phosphate uptake rates and the affinity for phosphate were higher for small (<0.8 mu m) organisms, compared with those >0.8 mu m. Phosphate uptake rates were relatively low, resulting in long phosphate turnover times (days), except in the surface waters south of 25 degrees S, which were also characterized by the highest uptake rates and affinity for phosphate, and the smallest total P pools observed along the transect. The organisms were found to realize their maximal phosphate uptake rates at ambient phosphate concentrations, suggesting the adequacy of the P supply to support the requirements of organisms. These findings suggest that inorganic P was 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 limited by P supply. The long turnover times generally observed in the Central Atlantic Ocean are in agreement with previous observations in oceanic systems elsewhere, suggesting that the observation that P is unlikely to be a limiting resource for planktonic growth can be extrapolated to most of the open ocean. The combined rate of P excretion from planktonic organisms and their microbial grazers as dissolved organic phosphorus (DOP) represented 75% of the total phosphate uptake. This DOP does not reach a sufficient accumulation as to drive an important downward flux of DOP, which represents a loss of only 9% of the P inputs into the biogenic layer. Hence, the high P uptake rate of the planktonic community in the Central Atlantic provides P in excess to support primary production, leading to a release as DOP, which appears to be rapidly recycled in the biogenic layer, thereby maintaining an adequate P supply to fuel primary production.