Seasonality in carbon chemistry of Cochin backwaters

Abstract Seasonality in carbon chemistry of Cochin backwaters, Southern India, was investigated between 2018 and 2019. Dissolved inorganic carbon (DIC) showed strong seasonal variations. Lowest DIC was observed during the Southwest Monsoon (SWM), in conjunction with low salinity in surface waters, suggesting strong freshwater influence. The maximum concentration of partial pressure of carbon dioxide in water (pCO2w) was recorded from polluted waters of Vembanad Lake ( ∼ 16,000 μ atm). Excluding the SWM, the inner most stations (freshwater) showed lower pCO2w levels compared with the outermost (estuarine) ones. With regard to sampling stations, all the environmental properties, except silicate and phosphate, exhibited significant variation, pointing to large spatial heterogeneity across the stations. Salinity showed a strong correlation with dissolved inorganic carbon within the Cochin backwaters. High pH and low pCO2w observed in some of the inner most stations indicates role of pH in carbonate speciation. Our study indicates large seasonal fluctuation in biogeochemical parameters and strong heterogeneity between individual stations which therefore necessitates development of local biogeochemical models for better understanding of carbon budget in these waters.