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 ($f_{{{\text{CO}}_{ 2} }}$) 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 $f_{{{\text{CO}}_{ 2} }}$ 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 $f_{{{\text{CO}}_{ 2} }}$ distribution was established during the study, although no significant correlation was found between chlorophyll-a and $f_{{{\text{CO}}_{ 2} }}$ (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 $f_{{{\text{CO}}_{ 2} }}$ ($\Updelta f_{{{\text{CO}}_{ 2} }}$) ranged from −274 to 69 μatm, with an average seaward flux of −10.5 ± 12.6 μmol m⁻² h⁻¹. The $\Updelta f_{{{\text{CO}}_{ 2} }}$ and hence the air–sea CO₂ exchange was primarily regulated by the variation in sea surface $f_{{{\text{CO}}_{ 2} }}$, since atmospheric $f_{{{\text{CO}}_{ 2} }}$ varied over a comparatively narrow range of 361.23–399.05 μatm.