Dinoflagellate cyst distribution in sediments of western Bay of Bengal: Role of sea surface conditions

Abstract

Bay of Bengal is a shallow marine basin exposed to seasonal fluctuation in precipitation and run-off. Thus, salinity, temperature, nutrient and productivity vary along the latitudinal and onshore-offshore gradient in the Bay of Bengal. These parameters directly affect primary productivity, including that of dinoflagellate cysts. The spatial distribution of organic walled dinoflagellates cyst is studied in the 50 surface samples of eight transects from the western Bay of Bengal, to infer variation in the environmental conditions in the region. The cyst diversity consists of 40 taxa with an overall high dominance of protoperidinioid cysts over gonyalaucoides. Based on the study distinct onshore and offshore dinocyst assemblages are identified. The high productive coastal upwelling region is characterised by the dominance of gonyalaucoid dinocyst species Bitectatodinium spongium (Zonneveld 1997) Zonneveld and Jurkschat 1999 indicating its adaptability to the fluctuating salinity and high nutrient conditions. The outer shelf to middle slope regions of western Bay of Bengal is dominated by the protoperidinioid species. This is in contrast to the gonyalaucoid dinocyst assemblages of the outer shelf to middle slope regions in other oceans. Since protoperidinioids prefer high nutrient and reducing environmental conditions for growth and preservation, their presence suggest high nutrient availability due to the river discharge and low oxygen condition in the slope region in Bay of Bengal. Statistical analyses of the sea surface parameters and dinoflagellate cyst abundance data indicate that salinity and silicates as a major source of nutrient played a major role in the distribution of dinoflagellate cyst in the Bay of Bengal.