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

The distribution of the fugacity of CO₂ (f_co2) 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 km² 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 atmospheric variables were measured in order to study their role in controlling the CO₂ flux. Surface water f_co2 ranged between 111 and 459 μatm 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 presence of non-carbonate alkalinity. An overall net biological control on the surface f_co2 distribution was established during the study, although no significant correlation was found between chlorophyll-a and f_co2 (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 F_co2) (Δf_co2) ranged from -274 to 69 μatm, with an average seaward flux of -10.5 ± 12.6 μmol m^-2 h^-1. The Δf_co2 and hence the air-sea CO₂ exchange was primarily regulated by the variation in sea surface f_co2, since atmospheric f_co2 varied over a comparatively narrow range of 361.23-399.05 μatm.