LAUSR

Abstract Seasonal variability in fresh water influx and ingression of sea water establish a unique transition zone in Asia's largest coastal lagoon – the Chilika lake. The seasonal variability in water and sediment input has significant impacts on the ecosystem influencing the rate of CO2 sequestration. We have assessed the impact of seasonal variability on CO2 sequestration using foraminiferal analysis and grain size data end member modeling. Lake floor sediment samples were collected seasonally, besides in situ measurements of physicochemical parameters of the lake bottom water at fixed stations influenced by seasonally variable fresh and saline water influx. Low electrical conductivity, pH of bottom water and absence of calcareous foraminifera suggest that the region proximal to the river mouth has fresh water ecosystem and has very limited role in CO2 sequestration. The region near to the sea mouth experiences high energy condition and seasonally changes from brackish to marine water ecosystem are ideal for diversification and opportunistic populations of foraminifera, but not ideal for flourishing (high population) of calcareous foraminifera. Inter-mixing of fresh and saline sea water develops fresh to brackish water conditions with medium energy condition in the interior region. These locations are away from the river and sea mouths and have high calcareous foraminifera abundances, but low diversity along with high bottom water pH suggesting seasonal variations in the sinking of CO2. Like other shallow coastal areas, Ammonia beccarii and Ammonia tepida are the dominant calcareous species, but their smaller size suggests a seasonal stressed condition. The opening of new and artificial sea mouth has increased the inflow of sea water into the lagoon that maintains its brackish water conditions, suggested by significant increase in calcareous foraminifera abundance. The sinking of these calcareous foraminifera may enhance the rate of CO2 sequestration and reduce degassing of dissolved carbon carried by the fresh water to the atmosphere.