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 a nd the Red Sea was studied during the SW- (May-August 1992) and NE-mon soon (January-February 1993). The diversity in physical and biological characteristics of the regions and seasons is reflected in a broad ra nge of both phyto- and bacterioplankton production. During the SW-mons oon, the Somali current showed highest bacterial production (up to 849 mgCm(-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 (aver age 225 mgCm(-2) day(-1)). Depth profiles of the upper 300 m in genera l showed a subsurface maximum in bacterial abundance and production at 20-70m depth. Heterotrophic activity and primary production were clos ely correlated, indicating the dependence of bacterioplankton on local phytoplankton-derived organic carbon and their ability to adapt quick ly to changes in the environment. The bacterial carbon demand in the u pper 300 m of the water column was largely supplied by phytoplankton p roduction in the euphotic zone. Bacterial production was 18+/-7% (aver age+/-S.D.) of primary production. Assuming an assimilation efficiency of 50% for marine bacteria, they consumed up to half of the carbon pr oduced by the phytoplankton. Cycling of carbon within the euphotic zon e appears to be achieved by intense grazing by (micro)zooplankton and subsequent remineralization. (C) 1997 Elsevier Science Ltd.