Assessing the effect of riverine discharge on planktic foraminifera: A case study from the marginal marine regions of the western Bay of Bengal

Abstract

Planktic foraminifera are amongst the most widely used paleoclimatic proxies. The application of foraminifera for paleoclimatic reconstruction requires knowledge of the ecological preference of species. Here, we report the surface distribution and ecology of planktic foraminifera from 99 core top samples collected from the riverine influx dominated western Bay of Bengal. The riverine influx reduces the planktic population on the shelf, in its immediate vicinity. Incidentally, the population increases on the slope in front of the major river mouths. A total 30 species belonging to 14 genera were found. Globigerinita glutinata is the most abundant species in the western Bay of Bengal, followed by Globigerina bulloides. Globigerina bulloides is abundant on the outer shelf, a region influenced by upwelling during southwest monsoon. The negative correlation of Neogloboquadrina dutertrei with sea surface salinity is attributed to the chlorophyll maximum due to the density gradient created by the hyposaline surface water. Globigerinoides ruber were abundant (>10%) at stations with higher salinity (>32 psμ). The mixed layer dwelling Trilobatus sacculifer are abundant on the slope. The increased relative abundance of Globigerinella siphonifera in shallow waters is attributed to the low salinity freshwater plume. The negative correlation of Globorotalia menardii with thermocline temperature and positive correlation with thermocline salinity suggests its cold water, high salinity habitat. We conclude that the freshwater and sediment influx and associated changes strongly modulate planktic foraminiferal distribution. The findings will help in better application of temporal changes in planktic foraminiferal population to reconstruct past environmental conditions from the Bay of Bengal.