Dynamics of inorganic phosphorus in pelagic communities of the Sea of Okhotsk

Abstract

Inorganic phosphorus uptake and regeneration in the Okhotsk Sea waters were investigated in July-August 1994 with the use of radioisotopic techniques. The rates of PO4-P uptake by microplankton in the upper mixed layer were between 1.5 and 6.6 μg P l-1 day-1 (average 2.75) in areas of diatom dominance, and between 0.68 and 1.68 μg P l-1 day-1 (average 1.16) in areas of intense warming and summer phytoplankton minimum. The residence time of PO4-P standing stock in water at different stations varied between 1.5 and 24 days (mean 9 days). The share of bacterioplankton contributing to total PO4-P uptake was ∼50% in areas of the summer phytoplankton minimum and 20-30% in areas of diatom dominance. The PO4-P regeneration rate was measured first time experimentally in the temperate sea. Its rates varied from 0.30 to 1.65 μg P l-1 day-1. In areas of diatom dominance, it compensated with 30-60% of PO4-P uptake. In zones of summer phytoplankton minimum and in the layers of deep chlorophyll maxima at 10-25 m depths, the PO4-P regeneration rate often exceeded its uptake. Primary phytoplankton production correlated well with PO4-P uptake values in the upper mixed layer, while no correlation was found between primary production and the ambient PO4-P content in water.