Abstract
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°N to 36° S). Phosphate uptake rates and the affinity for phosphate were higher for small (<0.8 μm) organisms, compared with those >0.8 μm. Phosphate uptake rates were relatively low, resulting in long phosphate turnover times (days), except in the surface waters south of 25°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°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.