LAUSR

Abstract The biogeochemical characteristics of Mahanadi estuarine system in eastern India were investigated to understand the seasonality in carbon parameters and their influence on sea–air CO2 gradient. In general, the stations close to the estuary had higher salinity and total alkalinity; and lower pCO 2 (water) relative to the inner most stations. In contrast, the riverine stations had low pH and high pCO 2(water) and lower total alkalinity. The chlorophyll a values ranged between 0.27 to 11.22 mg m−3 during this investigation. We found that the seasonal oscillations in sea–air CO2 gradient ( Δ pCO2 = pCO 2(water) –pCO 2(air) ) were negative during winter and positive during summer, wherein the estuary acted as a weak source. The average air–water CO2 flux density values varied between −5.65 to 13.11  μ mol m−2 h − 1 throughout the study period. Two-way ANOVA showed strong seasonal control on environmental variables. The relationships observed between pH and salinity with pCO2 suggest that fluvial inputs of nutrients and the accompanying heterotrophic processes may possibly control the sea–air CO2 gradient in the surface waters of the Mahanadi estuary.