Seasonality and variability of coccolithophore fluxes in response to diverse oceanographic regimes in the Bay of Bengal: Sediment trap results

Abstract

A study on coccolithophores from the sediment trap samples, collected over a period of one year (January 1992–December 1992) from three trap locations in the Bay of Bengal (BOB) were used to understand their production and export processes in relation to diverse oceanographic regimes associated with the reversing monsoons. A total of 25 species of coccolithophores were identified, which include some of the ecologically important species: viz: Gephyrocapsa oceanica, Umbilicosphaera sibogae, Florisphaera profunda, Umbellosphaera irregularis, Emiliania huxleyi, Oolithotus antillarum, U. tenuis, Helicosphaera carteri and Calcidiscus leptoporus. The total coccolithophore fluxes increased 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 at all trap locations thus inferring its preference for oligotrophic conditions caused 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 during the spring intermonsoon. The peak fluxes observed 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. The interannual variability of coccolithophore assemblages between the years 1992 and 1994 may be attributed to the prevalence of ENSO (El Nino–Southern Oscillation) and negative IOD (Indian Ocean Dipole) during 1992 and moderate ENSO and positive IOD with different ocean dynamics in 1994. Interestingly, dissolution was encountered as evidenced by the presence of corroded coccoliths. Similarly, mechanical breakage of coccoliths was also observed. It is believed that the biologically mediated processes were the main factors responsible for both the dissolution and mechanical breakage of coccoliths.