BIOMASS AND PRODUCTION OF HETEROTROPHIC BACTERIOPLANKTON IN THE OCEANIC SUB-ARCTIC PACIFIC

As part of the Subarctic Pacific Ecosystem Research (SUPER) program, w e measured the abundance and biomass production of heterotrophic bacte rioplankton in the subarctic Pacific and compared these parameters wit h those of phytoplankton during four cruises in 1987 and 1988. Bacteri al biomass was about equal to phytoplankton biomass during all cruises . Based on rates of bacterial biomass production and assuming a growth efficiency of 50%, we estimate that heterotrophic bacteria consumed 1 0% (June 1987) to 24% (August 1988) of primary production in the eupho tic zone. These percentages are low compared with other aquatic ecosys tems, apparently due to low bacterial growth rates (<0.1 day-1) in the subarctic Pacific. In contrast, phytoplankton growth rates were much higher (0.1-0.8 day-1). Bacterial growth rates were limited by the sup ply of dissolved organic matter and temperature. Even with these low g rowth rates, however, bacterial biomass and rates of biomass productio n increased by 2-5-fold in May and August 1988, changes that were not obviously related to corresponding changes in phytoplankton biomass no r primary production. Heterotrophic bacterioplankton constitutes a lar ge reservoir of carbon and nitrogen that needs to be considered in mod elling ecosystem dynamics of the subarctic Pacific.