Phosphorus dynamics in the North Pacific subtropical gyre

Phosphorus (P) dynamics were studied during several research cruises to Stn ALOHA and in the 'Climax region' of the North Pacific subtropical gyre (NP SG) in 1996-1997. The aim of this study was to: (1) investigate the coupled uptake and regeneration of inorganic phosphate (Pi) and the production of dissolved organic phosphorus (DOP), (2) quantify the size of the biological ly available P (BAP) pool, and (3) estimate the relative bioavailability of select organic P compounds to the natural microbial assemblages. At all st ations, the microbial community was dominated by prokaryotes (>99.5% of tot al cell numbers); Prochlorococcus spp. was the dominant pigmented group (>9 7% by numbers), comprising 20 to 30% of the total prokaryotic population. P hosphate uptake rates were 3.0 to 8.2 nM d(-1) (median = 3.5 nM d(-1)) and P pool turnover times ranged from 2 to 40 d (median = 9 d). The BAP pool ge nerally exceeded the Pi pool, suggesting rapid turnover of at least a porti on of the much larger DOP pool. The net production of DOP was approximately 10 to 40% of the net P uptake. Both the dissolved and the particulate orga nic matter pools were enriched in carbon (C) and nitrogen (N) relative to P , compared to the Redfield molar stoichiometry of 106C:16N:1P. The half-sat uration constant, K-m, values for P-i uptake were higher than the ambient P i pool concentrations, and uptake rates were positively correlated with exo genous P-i additions over the range tested (P-i = 25 to 250 nM). The bioava ilabilities of exogenous adenine and guanine nucleotides were generally hig her than other organic P compounds we tested. The net P-i regeneration rate from nucleotides was up to 50 times higher than the net P-i uptake rates, indicating a large potential for the regeneration of P-i from specific orga nic compounds. These P pool dynamics observed in the NPSG are consistent wi th a microbial community currently under P control.