CO2 effluxes from an urban tidal river flowing through two of the most populated and polluted cities of India

Urbanized rivers flowing through polluted megacities receive substantial amount of carbon from domestic sewage and industrial effluents which can significantly alter the air-water CO₂ flux rates. In this regard, we quantified the partial pressure of CO₂ in the surface water (pCO₂(water)), air-water CO₂ fluxes, and associated biogeochemical parameters in the Hooghly River, India, flowing through two of the most polluted cities of the country, Kolkata and Howrah, over a complete annual cycle during spring tidal phase (SP) and neap tidal phase (NP). This urbanized part of Hooghly River was always supersaturated with CO₂ having an annual mean pCO₂(water) and air-water CO₂ flux of ~ 3800 μatm and ~ 49 mol C m⁻² year⁻¹, respectively. Significant seasonal variability was observed for both pCO₂(water) and air-water CO₂ flux (pre-monsoon, 3038 ± 539 μatm and 5049 ± 964 μmol m⁻² h⁻¹; monsoon, 4609 ± 711 μatm and 7918 ± 1400 μmol m⁻² h⁻¹; post-monsoon, 2558 ± 258 μatm and 4048 ± 759 μmol m⁻² h⁻¹, respectively). Monthly mean pH and total alkalinity varied from 7.482 to 8.099 and from 2437 to 4136 μmol kg⁻¹, respectively, over the annual cycle. pCO₂(water) showed significant positive correlation with turbidity and negative correlation with electrical conductivity and gross primary productivity (GPP). High water discharge could have facilitated high turbidity, especially during the monsoon season, which led to depletion in GPP and enhancement in pCO₂(water) which in turn led to very high CO₂ effluxes. The CO₂ efflux rate in this urbanized riverine stretch was substantially higher than that observed in previous studies carried out in the less urbanized estuarine stretch of Hooghly. This indicates that the presence of highly urbanized and polluted metropolis potentially enhanced the pCO₂(water) and CO₂ effluxes of this river. Similar observations were made recently in some Asian and Australian urban rivers.