Radiolarian fluxes from the southern Bay of Bengal: sediment trap results

A study of radiolarian fluxes collected during 1991–93 from time-series sediment traps deployed at 1071 and 3010 m water depth in the southern Bay of Bengal (SBBT) yielded 40 species/groups of radiolarians. Among the order Polycystina, the species of sub-order Spumellaria were by far the most abundant (∼95%) followed by sub-order Nassellaria (5%). This is contrary to reports from the Atlantic and Pacific Oceans and is attributed to the prevailing hyposaline condition resulting from the monsoonal rainfall. Higher radiolarian fluxes occurred during March–May, when moderate salinity and a high sea surface temperature (SST) regime prevailed at the trap site. R-mode cluster analysis of the radiolarian flux data revealed three assemblages represented by the cooler (A) and warmer (C) surface dwelling fauna (0–50 m) dominated by spumellarians, and a deeper dwelling (B) sub-surface fauna (50–100 m) associated with deep dwelling (>100 m) nassellarian species. Spongaster tetras tetras, a surface water radiolarian species, exhibited its preference for high SST and moderate salinity conditions during the pre-monsoon season (March–May). Radiolarian fluxes responded to seasonal changes in SST and salinity variations due to the monsoonal precipitation, and the freshwater runoff from the Indian rivers causing a hyposaline condition in the Bay of Bengal. Results imply that the radiolarian assemblages in the down core data may reveal the monsoonal history in the geological past.