Assessing the Effect of Riverine Discharge on Planktic Foraminifera: A Case Study from the Western Bay of Bengal

The planktic foraminifera are frequently used to reconstruct water column properties in the past. The application of planktic foraminifera to reconstruct water column properties requires a precise understanding of their ecology. Here, we assess the effect of huge riverine influx on the distribution of planktic foraminifera. The surface distribution and ecology of different planktic foraminifera is documented from 99 core top samples collected from the rivering influx dominated western Bay of Bengal. The low absolute abundance of planktic foraminifera in the shelf region is attributed to dilution by high terrestrial input. Globigerina bulloides is abundant in the outer shelf region suggesting 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 prefers warmer temperature and higher salinity. The mixed layer dwelling Globigerinoides sacculifer with sac are abundant on the lower slope, whereas the ones without sac are comparatively abundant on the upper slope. The findings support the earlier observations of the shift of G. sacculifer population to relatively colder, deeper water for reproduction when it secrets the terminal sac like chamber. The increased relative abundance of Globigerinilla aequilateralis in shallow waters is attributed to low salinity freshwater plume. Although the dissolution resistant Pulleniatina obliquiloculata is rare in deeper waters, a significant drop in planktic foraminiferal population at depths >2000 m clearly suggests it to be the foraminiferal lysocline in the western Bay of Bengal. 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 huge freshwater and sediment influx and associated changes strongly modulate planktic foraminiferal distribution in the western Bay of Bengal. The findings will help in a better application of temporal changes in planktic foraminiferal population to reconstruct past environmental conditions from the Bay of Bengal.