Production in the Sea of Okhotsk

Abstract

Primary production, microbial production and the density of planktonic microheterotrophs were estimated at 40 stations in the Okhotsk Sea in July-August 1992 during the seasonal phyto- plankton minimum. The primary production by phytoplankton remained rather high even during this minimum. At most stations it was >0.6-0.8 g m-2 day-1, and in leftover patches of spring diatom 'bloom' it reached >5 g C m-2 day-1. The deep maxima of phytoplankton at the upper boundary of the seasonal thermocline were an ordinary phenomenon. The depth of the euphotic zone was normally 30-50 m in the open sea and 12-25 m at the shelf stations. Any correlations between the phosphate contents in the upper mixed layer and primary production were absent at the stations. There was no adaptation of the phytoplankton to the light deficiency in deep maxima layers. The total numbers of bacterioplankton were 1-1.5 3 106 ml-1 and its biomass was close to 100 mg m-3 in the open sea. All these numbers were 2-3 times greater at the shelf stations. In deep waters, the bacterio- plankton biomass decreased to 10-40 mg m-3. The microbial production in the upper layer was high, at 50-100 mg m-3, decreasing 50-100 times in the deep waters. The numbers of ciliates in the upper water layer varied from 3 to 6 3 103 l-1 and were 1.5-2 times greater than in the shelf areas. Ciliate biomass was 60-100 mg m-3 in the upper mixed layer, and per square metre varied to 1.5-2.5 g. The dominant ciliate taxa belonged to the naked oligotrichid genera Strombidium and Tontonia. Tentative calculations were made of the basin's annual primary production and for the analysis of energy balance in the ecosystem.