Abundance and productivity of bacterioplankton in relation to seasonal upwelling in the northwest Indian Ocean

The role of bacterioplankton in the Somali Current, the Gulf of Aden and the Red Sea was studied during the SW- (May–August 1992) and NE-monsoon (January–February 1993). The diversity in physical and biological characteristics of the regions and seasons is reflected in a broad range of both phyto- and bacterioplankton production. During the SW-monsoon, the Somali current showed highest bacterial production (up to 849 mgC m−2 day−1) in regions with enrichment of the surface waters by upwelling of cold, nutrient-rich, deep water. In contrast, the Gulf of Aden and the Red Sea were most productive during the NE-monsoon (average 225 mgC m−2 day−1). Depth profiles of the upper 300 m in general showed a subsurface maximum in bacterial abundance and production at 20–70 m depth. Heterotrophic activity and primary production were closely correlated, indicating the dependence of bacterioplankton on local phytoplankton-derived organic carbon and their ability to adapt quickly to changes in the environment. The bacterial carbon demand in the upper 300 m of the water column was largely supplied by phytoplankton production in the euphotic zone. Bacterial production was 18 ± 7% (average ± S.D.) of primary production. Assuming an assimilation efficiency of 50% for marine bacteria, they consumed up to half of the carbon produced by the phytoplankton. Cycling of carbon within the euphotic zone appears to be achieved by intense grazing by (micro)zooplankton and subsequent remineralization.