Alkaline phosphatase activities in the central Atlantic Ocean indicate large areas with phosphorus deficiency

The activity of the enzyme alkaline phosphatase (APA) was studied along 2 latitudinal transects in the central Atlantic (28° S to 28° N) and compared to the distribution of nutrient concentrations, planktonic biomass and other variables reflecting community P status. Using 3-0-methyl fluorescein phosphate (MF-P), APA was measured fluorometrically in 2 size fractions (<0. 8 and <150 μm) and in the dissolved fraction (i.e. that fraction passing through 0.2 μm filters). Significant APA (p < 0.05) was recorded over extensive areas of the central Atlantic, ranging from 4 to 50 nmol MF-P l ^-1 h^-1. Most activity arose from the free dissolved fraction, followed by the <0.8 μmn size fraction. The relatively low APA in the central Atlantic is due to the low biomass that characterises most of the area covered by the transects. Nevertheless, the specific activity per unit biomass was high in both the north and south Atlantic subtropical gyres in this area. Along the transects, APA displayed an inverse relationship to the calculated upward turbulent nutrient fluxes. The results support the existence of persistent P-limited conditions in much of the central Atlantic Ocean. This is consistent with stoichiometric data and estimated rate processes and fluxes in the area, indicating that P regeneration from dissolved organic phosphorus may play an important role in the central Atlantic, allowing efficient utilisation of P in the biogenic layer and avoiding P loss through the diffusive vertical flux of dissolved organic matter that preferentially removes C and N.