Dominance of dinoflagellates in micro- zooplankton community in the oceanic regions of the Bay of Bengal and the Andaman Sea

Studies on microzooplankton, phytoplankton biomass (chlorophyll a) and production were conducted at five oceanic stations in the Bay of Bengal and Andaman sea onboard FORV Sagar Sampada during intermonsoon (March 2001). Throughout the study protozoans dominated the microzooplankton community and their contribution ranged between 60 and 100% (avg 88%). Protozoans consisted mainly of ciliates and dinoflagellates. Tintinnids comprised 36 species belonging to 18 genera and 76 species of dinoflagellates belonging to 24 genera were recorded. Most of the genera showed their presence throughout the upper 150 m water column except four genera, two each belonging to dinoflagellates and tintinnids. Amphisolenia and Exuviaella (Dinoflagellates) were concentrated in the upper 20 m, whereas Salpingella and Salpingacantha (Tintinnids) were restricted to 75-100 m. Contribution of dinoflagellates to the total density of microzooplankton was as large as 80% (avg 59%). In this paper, the dominance of dinoflagellates is discussed with reference to different modes of nutrition of this group. Combining many nutritional strategies (mixotrophy and heterotrophy with symbiosis) may give dinoflagellates a competitive advantage over strict phototrophs or heterotrophs and may be the possible reason for their dominance in the study area.