Basin-scale geographic patterns of bacterioplankton biomass and production in the subarctic Pacific, July-September 1997

Bacterial biomass and production rate were measured in the surface (0-100 m) and mesopelagic layers (100-1,000 m) in the subarctic Pacific and the Bering Sea between July-September, 1997. Depth profiles were determined at stations occupied in oceanic domains including the subarctic gyres (western, Bering Sea, and Gulf of Alaska) and a boundary region south of the gyres. In the surface layer (0-100 m), both bacterial biomass and production were generally high in the western and Bering Sea gyres, with the tendency of decrease toward east. This geographic pattern was consistent with the dominant regime of phytoplankton biomass at the time of our survey. A significant portion of variation in bacterial production was explained by the concentration of chlorophyll a (r2 = 0.340, n = 60, P < 0.001) and, to the greater extent, by the concentration of semilabile total organic carbon (SL-TOC = TOC at a given depth-TOC at 1,000 m, r2 = 0.488, n = 59, P < 0.0001). Temperature significantly improved the regression model: Temperature and chlorophyll jointly explained 60% of variation in bacterial production. These results support the hypothesis that bacterial growth is largely regulated by the combination of temperature and the supply of dissolved organic carbon in subarctic surface waters. In the mesopelagic layer (100-1,000 m), the geographic pattern of bacterial production was strikingly different from the surface phytoplankton distribution: The production was high in the boundary region where the phytoplankton biomass was lowest. Bacterial growth appeared to be largely controlled by the supply of organic carbon, as indicated by the strong dependency of bacterial production on SL-TOC (r2 = 0.753, n = 75, P < 0.0001). The spatial uncoupling between surface phytoplankton and mesopelagic bacterial production suggests that the supply rate of labile dissolved organic carbon in the mesopelagic zone does not simply reflect the magnitude of the particulate organic carbon flux in the subarctic Pacific.