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

As part of the Subarctic Pacific Ecosystem Research (SUPER) program, we measured the abundance and biomass production of heterotrophic bacterioplankton in the subarctic Pacific and compared these parameters with those of phytoplankton during four cruises in 1987 and 1988. Bacterial 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 10% (June 1987) to 24% (August 1988) of primary production in the euphotic zone. These percentages are low compared with other aquatic ecosystems, 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 supply of dissolved organic matter and temperature. Even with these low growth rates, however, bacterial biomass and rates of biomass production increased by 2-5-fold in May and August 1988, changes that were not obviously related to corresponding changes in phytoplankton biomass nor primary production. Heterotrophic bacterioplankton constitutes a large reservoir of carbon and nitrogen that needs to be considered in modelling ecosystem dynamics of the subarctic Pacific.