| Summary: | 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. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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