AME 22:185-198 (2000)  -  doi:10.3354/ame022185

Phosphorus dynamics in the North Pacific subtropical gyre

Karin Björkman*, Angie L. Thomson-Bulldis**, David M. Karl

Department of Oceanography, School of Ocean & Earth Science & Technology (SOEST), University of Hawaii, Honolulu, Hawaii 96822, USA
*E-mail: **Present address: College of Education, University of Washington, Seattle, Washington 98195, USA

ABSTRACT: Phosphorus (P) dynamics were studied during several research cruises to Stn ALOHA and in the ŒClimax region¹ of the North Pacific subtropical gyre (NPSG) 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 biologically available P (BAP) pool, and (3) estimate the relative bioavailability of select organic P compounds to the natural microbial assemblages. At all stations, the microbial community was dominated by prokaryotes (>99.5% of total cell numbers); Prochlorococcus spp. was the dominant pigmented group (>97% by numbers), comprising 20 to 30% of the total prokaryotic population. Phosphate 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 generally exceeded the Pi pool, suggesting rapid turnover of at least a portion 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 organic 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-saturation constant, Km, values for Pi uptake were higher than the ambient Pi pool concentrations, and uptake rates were positively correlated with exogenous Pi additions over the range tested (Pi = 25 to 250 nM). The bioavailabilities of exogenous adenine and guanine nucleotides were generally higher than other organic P compounds we tested. The net Pi regeneration rate from nucleotides was up to 50 times higher than the net Pi uptake rates, indicating a large potential for the regeneration of Pi from specific organic compounds. These P pool dynamics observed in the NPSG are consistent with a microbial community currently under P control.

KEY WORDS: Phosphate uptake rates · Turnover rates · Bioavailable phosphorus · Bacteria · North Pacific

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