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Microbial processes in the subarctic northeast Pacific : bacterial standing stocks, rate processes, and controls Sherry, Nelson Daniel


During 11 cruises 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), heterotrophic bacterial abundance, productivity, and respiration rates were measured. While values from below the euphotic zone (-60 - 4200 m) are reported, detailed analysis is focused primarily on the euphotic zone. Within the euphotic zone, bacterial biomass in the winter was -50% of the spring and summer values. Bacterial respiration rates in the summer increased > 10-fold at P4 while showing much less change at the more oceanic stations. Bacterial productivity, measured as [ H]-thymidine and [14C]-leucine incorporation rates, was lowest in the winter with little spatial variability. Productivity increased 4 to 10-fold in spring at P4. Between 1995 and 1998, the average summer bacterial abundance along line-P decreased by -50% while bacterial productivity decreased by - 90%. Factors for converting the incorporation of radiolabeled substrate to bacterial productivity were determined empirically from grow-out dilution experiments. The empirical conversion factors for both thymidine and leucine were positively correlated to in situ cell abundance. A grow-out simulation model was developed and used to quantify the uncertainties associated with various conversion factor calculation methods leading to a more robust, non-linear cumulative method for calculating conversion factors. Roughly 10%o of the mixed-layer data showed extremely high bacterial productivity which was associated with physical features or fronts. Seasonal changes in bacterial abundance, productivity, and growth rate were driven predominantly by changes in the supply rate of phytoplankton derived dissolved organic carbon (DOC), and by changes in the dilution of surface water by deeper water (deepening of the mixed layer and/or changes in upwelling). Regional changes in bacterial abundance, productivity, and growth rate during the summer were driven predominantly by differences in the availability of limiting mineral nutrients and changes in the rates of loss processes. Experimental manipulations demonstrated that bacterial productivity was almost always limited by the availability of labile DOC. However, bacterial productivity in the late summer was further limited at P4 in 1996 by the availability of nitrogen, and at OSP in 1997 by the availability of iron.

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