Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layer of the Bering Sea during the late spring-early summer period ranged between 1 and similar to 4 x 10(6) ml(-1) (biomass 10-40 mg C m(-3)). In the northern Pacific, along 47-52 degrees N, the corresponding characteristics of the bacterioplankton density in the upper mixed water layer were: total number 1-2 x 10(6) cells ml(-1) and biomass 15-46 mg C m(-3). Below the thermocline at 50-100 m, the density of bacterioplankton rapidly decreased. At 300 m depth, it stabilized at 0.1-0.2 x 10(6) cells ml(-1). The integrated biomass of bacterioplankton in the open Bering Sea ranged between 1.2 and 3.6 g C m(-2) (wet biomass 6-18 g m(-2)). Its production per day varied from 2 to 23 mg C m(-3) days(-1) in the upper 0-100 m. The numerical abundance of planktonic ciliates in this layer was estimated to be from 3 to 10 x 10(3) cells l(-1), and in the northern Pacific from 0.4 to 4.5 x 10(3) l(-1). Their populations were dominated by naked forms of Strombidium, Strombilidium and Tontonia. Ln some shelf areas, up to 40% of the total ciliate population was represented by the symbiotic ciliate Mesodinium rubrum. The data on the integrated biomass of basic groups of planktonic microheterotrophs are also presented, and their importance in the trophic relationships in pelagic communities of subarctic seas is discussed.