Seasonal and spatial patterns of heterotrophic bacterial production, respiration, and biomass in the subarctic NE Pacific

Heterotrophic bacterial biomass, production, and respiration rates were measured during winter, spring, and summer in the subarctic NE Pacific from September 1995 to June 1997. Sampling took place on six cruises at five hydrographic stations along the east/west line-P transect from slope waters at P4 (1200 m depth) to the open-ocean waters at Ocean Station Papa (OSP) (4250 m depth). Interannual variability was small relative to seasonal and spatial variability. Biomass, derived from cell counts (assuming 20 fg C cell−1), was ca. 12 μg C l−1 in the winter and increased to 20–35 μg C l−1 in the spring and summer all along line-P. Bacterial production from [3H]-thymidine and [14C]-leucine incorporation rates was lowest in the winter (ca. 0.5 μg C l−1 d−1) with little spatial variability. Production increased 10-fold in spring at P4 (to ca. 4.5 μg C l−1 d−1). In contrast, only a 2-fold increase in bacterial production was observed over this period at the more oceanic stations. Rates of production in late summer were highest over the annual cycle at all stations ranging from ca. 6 at P4 to ca. 2 μg C l−1 d−1 at OSP. Bacterial (<1 μm size fraction) respiration, measured from dark-bottle O2 consumption over 24 or 48 h, was <10 μg C l−1 d−1 during the winter and spring. Respiration rates increased >10-fold to ca. 100 μg C l−1 d−1 at P4 in the summer, but, interestingly, did not increase from spring to summer at the more oceanic stations. Thus bacterial growth efficiency, defined as production/(production+respiration), decreased in the spring westwards from the slope waters (P4) to the open-ocean (OSP), but increased westwards in the summer. Bacterial production was highly correlated with temperature at OSP (r2=0.88) and less so at P4 (r2=0.50). The observed temporal and spatial trends presented in this study suggest that seasonal changes in bacterial biomass were greatly affected by changes in loss processes, that bacterial biomass is regulated by different processes than bacterial production, and that bacterial production alone, without respiration measurements, is not a robust proxy for bacterial activity in the subarctic NE Pacific.