Ocean Deoxygenation Conference | Kiel 2018

Hydrographic observations in the Cochin backwaters (CBW), southwest coast of India during the summer monsoon delineated the spreading of an unusually anoxic water mass in the bottom characterized by intense denitrification and formation of hydrogen sulphide, which have not been previously reported from any tropical estuaries. The coastal zone of western India experiences moderate upwelling during this period. The oxygen deficient (~0.5 ml.l-1 or 22 µM) upwelled waters, as it intrudes into the estuary through the bottom, is quickly depleted in the oxygen content to near-zero levels owing to the excessive demand for the oxidation of organic matter. Increased river inputs countering the penetration of upwelled waters cause strong stratification and poor ventilation leading to reducing conditions near the bottom. It was also observed that increased latter inputs rich in organic matter have resulted in the development of anoxic conditions (O2=0), 2-6 kms inside the estuary, where the concentration of hydrogen sulphide increased up to 12 µM. An increase in the nitrite concentrations (up to 2.4 µM) in the oxygen deficient waters compared to its levels at the Cochin inlet and corresponding to a decrease in the nitrate inputs indicated that denitrification was active. The intrusion of upwelled waters seems to be much stronger (up to ~16 kms inside the estuary) compared to its first report in 1968, which is attributed to a decline in the river discharge and deepening of the estuarine channels. The expansion of oxygen deficient zone and formation of hydrogen sulphide are anthropogenically induced and could be destructive to the biological productivity and an increased emission of greenhouse gases from this region.