Characterizing air–sea CO2 exchange dynamics during winter in the coastal water off the Hugli-Matla estuarine system in the northern Bay of Bengal, India

Abstract

The distribution of the fugacity of CO2 (fCO2) and air-sea CO2 exchange were comprehensively investigated in the outer estuary to offshore shallow water region (lying adjacent to the Sundarban mangrove forest) covering an area of *2,000 km 2 in the northern Bay of Bengal during the winter. A total of ten sampling surveys were conducted between 1 December, 2011 and 21 February, 2012. Physico- chemical variables like sea surface temperature (SST), salinity, pH, total alkalinity (TAlk), dissolved inorganic carbon (DIC) and in vivo chlorophyll-a along with atmo- spheric variables were measured in order to study their role in controlling the CO2 flux. Surface water fCO2 ranged between 111 and 459 latm which correlated significantly with the SST (r = 0.71, p \ 0.001, n = 62). Neither DIC nor TAlk showed any linear relationship with varying salinity in the estuarine mixing zone, demonstrating the significant pre- sence of non-carbonate alkalinity. An overall net biological control on the surface fCO2 distribution was established during the study, although no significant correlation was found between chlorophyll-a and fCO2 (water). The shallow water region studied was mostly under-saturated with CO2 and acted as a sink for atmospheric CO2. The difference between surface water and atmospheric fCO2 (DfCO2) ranged from -274 to 69 latm, with an average seaward flux of - 10.5 ± 12.6 lmol m -2 h -1 . The DfCO2 and hence the air- sea CO2 exchange was primarily regulated by the variation in sea surface fCO2, since atmospheric fCO2 varied over a com- paratively narrow range of 361.23-399.05 latm.