Seasonal and interannual trends in heterotrophic bacterial processes between 1995 and 1999 in the subarctic NE Pacific

Abstract

Abstract Heterotrophic bacterial abundance and productivity were measured in the euphotic zone over 4 years during winter, late spring, and late summer in the subarctic NE Pacific from September 1995 to February 1999. Sampling took place during 11 cruises at five hydrographic stations spaced 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). P4 exhibits a spring bloom and summer nutrient depletion, whereas OSP exhibits no spring bloom, consistently replete macronutrients, and iron limitation of primary productivity. Bacterial biomass, derived from cell abundance (assuming 10 fg C cell −1 ), ranged from an average of ∼6 μg C l −1 in the winter to 10 and 14 μg C l −1 in the spring and summer all along line- P . Factors for converting the uptake of radiolabeled substrate to bacterial productivity were determined empirically from dilution experiments. The empirically derived conversion factors for both thymidine and leucine were positively correlated to in situ cell abundance. Bacterial productivity estimated from [ 3 H]-thymidine and [ 14 C]-leucine incorporation rates was lowest in the winter (∼0.2 μg C l −1 d −1 ), with little spatial variability. Bacterial productivity increased 4–10-fold in spring at P4 compared with 0–4-fold at the more oceanic stations. Between 1995 and 1998, the average summer bacterial abundance all along line- P decreased by ∼50%, while bacterial productivity decreased by ∼90%. These decreases reduced summer bacterial productivity and growth rates to nearly winter levels in both the open-ocean and slope waters during late summer 1998. The average bacterial productivity at OSP is similar to other open-ocean regions while bacterial abundance is slightly higher providing for relatively lower growth rates and inferring lower associated loss rates.