AME 33:117-126 (2003)  -  doi:10.3354/ame033117

ABSTRACT: The carbon flux through bacterioplankton under oligotrophic conditions was studied during summer in the southern Bay of Biscay. An episode of coastal upwelling during our study allowed us to look at how bacterial metabolism responded to short-term environmental changes. Gross primary production (GPP) was significantly higher after the upwelling pulse; however, microbial respiration (R) exceeded the organic carbon fixation by primary producers (R > GPP), resulting in net heterotrophic balance during summer (net community production [NCP] < 0). Heterotrophic bacteria were the major contributors to bulk community respiration, accounting for 70% of the total respiration. The variability in both bacterial respiration and plankton respiration was largely attributable to differences in the dissolved organic carbon (DOC) concentration. The ratio of bacterial biomass to total phytoplankton biomass decreased after the upwelling pulse, whereas the bacterial production to primary production ratio increased as the productivity of the system increased. Bacterial production was always significantly lower than bacterial respiration, giving rise to a bacterial growth efficiency (BGE) lower than 10%. BGE responded to changes in phytoplankton production, increasing as the system became more productive. Short-term variations in the structure of the water column may have an important impact on bacterial processes within this oligotrophic system.

KEY WORDS: Bacterial respiration · Bacterial production · Bacterial growth efficiency · Gross primary production · Upwelling pulse · Oligotrophic period