Sources of organic matter and tracing of nutrient pollution in the coastal Bay of Bengal

Abstract

Stable isotopic composition of carbon (δ¹³C_POM) and nitrogen (δ¹⁵N_POM) in the particulate organic matter (POM) is used to identify sources of organic carbon and nutrients using monthly time-series observation in the coastal Bay of Bengal (BoB). The hydrographic structure indicates that the coastal BoB is influenced by coastal upwelling during March-May, advection of peninsular river discharge during June to September and glacial (Ganges) river discharge during October to December due to reversing of East India Coastal Currents (EICC). C/N ratios in POM were mostly higher values than Redfield ratio in the study region indicating possible contribution of terrestrial origin. Enriched δ¹³C_POM were found during March-May associated with coastal upwelling indicating major contribution of POM from the in situ production while lower values were noticed during June to September followed by October to December indicating influence of terrestrial sources. δ¹⁵N_POM displayed strong inverse relation with salinity and linear relation with Chl-a suggesting that anthropogenic nutrients from the land increased coastal phytoplankton biomass. δ¹⁵N_POM linearly decreased with increase in distance from the coast and reached to the typical offshore value of 6-6.5‰ indicating that terrestrial nutrients influence was spread up to a distance of 15 to 20 km from the coast in the study region. Our study suggested that coastal waters are influenced by terrestrial/anthropogenic nutrients and its impact can be noticed up to 15 to 20 km from the coast and its impact on Biodiversity Beyond National Jurisdiction may be negligible than hitherto hypothesized.

Keywords: Bay of Bengal; Coastal currents; Nutrients; Organic matter; Pollution; River discharge.