TROPHIC INTERACTIONS BETWEEN PICOPHYTOPLANKTON AND MICROZOOPLANKTON AND NANOZOOPLANKTON IN THE WESTERN ARABIAN SEA DURING THE NE MONSOON 1993

The grazing pressure of micro- and nanozooplankton on phytoplankton wa s estimated in serial dilution experiments in the northwestern Arabian Sea and its adjacent areas (the Somali Current, the Somali Basin, the Gulf of Aden and the southern Red Sea) during the NE monsoon 1992-199 3. Microzooplankton grazing rates (g) on total phytoplankton (analyzed as chi a) were generally exceeded by phytoplankton growth rates (g = 0.2 to 1.19 d(-1), mean 0.48 d(-1); mu = 0.52 to 1.12 d(-1), mean 0.72 d(-1)), resulting in an average daily consumption of 38 % of the phyt oplankton standing stock and 67 % of the primary production. Microzoop lankton grazing on 4 picophytoplankton groups (Prochlorococcus spp., S ynechococcus spp., and 2 picoeukaryotes) analyzed by flow cytometry sh owed growth (mu = 0.27 to 0.92 d(-1), mean 0.68 d(-1)) and grazing mor tality rates (g = 0.26 to 0.73 d(-1), mean 0.67 d(-1)) well in balance , with an average of 49 % of the standing stock and 102% of the primar y production grazed per day. Picophytoplankton growth and grazing mort ality rates increased dramatically when grazers >10 mu m were removed. These results suggest a control of the small grazers by larger ones ( trophic cascade) and a close coupling between picoautotrophic prey and small grazers. The trophic cascade within the microbial food web of t he nanoplankton encompasses 3 trophic levels: picoplankton - small HNF - larger flagellates and ciliates.