Phosphate uptake by phytoplankton in the central North Pacific Ocean

The central North Pacific Ocean (CNP) is an oligotrophic environment characterized by low concentrations of plankton biomass and of dissolved nitrogen and phosphorus compounds. The dynamics of phosphorus utilization was examined to test the hypothesis that phosphorus controlled phytoplankton growth rates. Rates of phosphate uptake were measured in natural assemblages using 33P-phosphorus and were compared with rates of 15N-nitrogen and 14C-carbon assimilation. Phosphate uptake was linear over 24 h. Turnover times for phosphate averaged 28 d in February, 19 d in June, and 31 d in December. Zooplankton regeneration accounted for 55 to 183% of the daily phytoplankton phosphate requirement. Growth rates of phytoplankton in the mixed layer, calculated from 33P-phosphorus uptake, were 9% of the maximum rate at ambient central North Pacific temperatures and averaged 0.14 doublings d −1. Assimilation ratios of C, N, and P suggested nitrogen-limited, rather than phosphorus-limited, growth. Physiological adaptions, such as low K m for phosphate uptake, light-independence of uptake, and derepressible alkaline phosphatase, may prevent the onset of phosphorus stress.