Characterizing air-sea CO_2 exchange dynamics during winter in the coastal water off the Hugli-Matla estuarine system in the northern Bay of Bengal, India

摘要

The distribution of the fugacity of CO_2 (f_(CO_2)) and air-sea CO_2 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 atmospheric variables were measured in order to study their role in controlling the CO_2 flux. Surface water f_(CO_2) ranged between 111 and 459 uatm 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_(CO_2) distribution was established during the study, although no significant correlation was found between chlorophyll-a and f_(CO_2) (water). The shallow water region studied was mostly under-saturated with CO_2 and acted as a sink for atmospheric CO_2. The difference between surface water and atmospheric f_(CO_2) (Δf_(CO_2)) ranged from -274 to 69 μatm, with an average seaward flux of -10.5 ± 12.6 μmol m~(-2) h~(-1). The Δf_(CO_2) and hence the air-sea CO_2 exchange was primarily regulated by the variation in sea surface f_(CO_2), since atmospheric f_(CO_2) varied over a comparatively narrow range of 361.23-399.05 μatm.