Temporal variation of coccolithophores in the Bay of Bengal

Coccolithophores in sediment trap samples collected over a period of one year (January 1992 - December 1992) from three trap locations in the Bay of Bengal were studied to understand their production and export processes in relation to diverse oceanographic regimes associated with the reversing monsoons. This study has yielded a total of 25 species of coccolithophores which included Gephyrocapsa oceanica, Umbilicosphaera sibogae, Florisphaera profunda, Umbellosphaera irregularis, Emiliania huxleyi, Oolithotus antillarum, U.tenuis, Helicosphaera carteri and Calcidiscus leptoporus species. The total coccolithophore fluxes showed a progressive increase from the northern to southern Bay of Bengal where, G. oceanica, U. sibogae and E.huxleyi were dominant species displaying seasonality with peak fluxes occurring during southwest (SW) and northeast (NE) monsoons reflecting their affinity for nutrient-rich waters brought in by river plumes, divergences and cyclonic eddies. U. irregularis, an indicator of oligotrophic/warm water, recorded peak abundance fluxes only during the spring intermonsoon period implying its preference for oligotrophic conditions resulted by increased Sea Surface Temperature (SST), stratification and lack of nutrient supply due to weak wind velocities. The deep dwelling species, F. profunda recorded high fluxes during the fall intermonsoon (October/November) in the northern and southern traps and during the spring intermonsoon in the central trap. The peak fluxes during the spring intermonsoon (April/May) in the central and southern traps indicated the prevalence of a deep nutricline when oligotrophic conditions prevailed at the surface. Interestingly, dissolution was encountered throughout the year as evidenced by the presence of corroded coccoliths in addition to fragmented ones attributed to biologically mediated processes.