Seasonal cycle of biogenic flux and associated upper ocean processes in the southern Bay of Bengal and equatorial Indian Ocean

Seasonal cycle of biogenic flux obtained from sediment trap at two locations, one in the southern Bay of Bengal (5°24’N, 86°46’E) and the other in the equatorial Indian Ocean (3°34’N, 77°46’E), were analysed to understand the factors that control them. The southern Bay of Bengal trap (SBBT) data is for a period of ten years from November 1987 to December 1997, while the equatorial Indian Ocean trap (EIOT) data is for one year period from January to December 1996. The climatological monthly mean biogenic flux in SBBT showed a strong seasonal variability while no such seasonal variability was observed in EIOT, though both the traps were located in close geographical proximity. In contrast, the monthly mean climatology of the satellite derived chlorophyll pigment concentrations showed well-defined seasonality with enhanced values during summer monsoon (June-September) at both these locations. However, the magnitude of the enhancement at EIOT location was much less. Based on the analysis of the wind, surface current, sea surface temperature, photosynthetic active radiation (PAR), diffuse attenuation coefficient (Kd490), and nutrient data we show that the upper ocean processes such as local Ekman pumping (upwelling) and advection played a dominant role in making available the most limiting nutrients to the oligotrophic upper ocean which resulted in enhancing the biogenic fluxes in the deep sea during summer monsoon at the SBBT. A comparison of the static stability at both SBBT and EIOT showed that the water column at EIOT location was more strongly stratified than that at SBBT. Hence the wind, which was less stronger at EIOT in comparison to SBBT, was unable to break the strong stratification at EIOT making it oligotrophic. Thus, lack of processes such as upwelling and strong wind-driven mixing at EIOT makes the waters oligotrophic supressing high biological productivity and leading to low biogenic flux.