SYNECHOCOCCUS AND ITS IMPORTANCE TO THE MICROBIAL FOODWEB OF THE NORTHWESTERN INDIAN-OCEAN

The abundance, distribution, size, biomass, growth and grazing-induced mortality of phycoerythrin (PE) rich chrococcoid cyanobacteria were studied during September-October 1986 in the Arabian Sea, the Gulf of Oman and the monsoonal upwelling region off the South East Arabian coast. Cyanobacteria were abundant (>10(7) cells l-1) through the region and particularly so (>10(8) cells l-1) in oligotrophic waters where they exhibited distinct subsurface concentration maxima that were situated above, but related to the depth of the chlorophyll maxima. Cell diameter increased from 0.7 mum in surface waters to 1.2 mum at depth. Standing stocks of cyanobacteria ranged up to 50 mugC 1-1, and accounted for up to 40% of the POC in oligotrophic stations indicating that Synechococcus constitutes an important trophic resource. Experimental investigations showed that cyanobacterial populations were growing fast, with specific growth rates of 0.5-1.0 day-1, while simultaneously experiencing high mortality due to microzooplankton grazing. Grazing rates varied between 0.3 and 1.2 day-1, indicating that 31-71% of the cyanobacteria were predated daily. Grazing and cyanobacterial growth were correlated, suggesting that Synechococcus production and its fate by microbial grazing activity were tightly coupled. Cyanobacteria are clearly a major component of a dynamic but well-balanced microbial foodweb present in oligotrophic regions of the northwest Indian Ocean.