Contrasting Seasonal Daytime CO2 Fluxes in Tropical Irrigation Tanks Under Variable Hydrological and Physicochemical Conditions

Ancient irrigation tanks are a defining component of Sri Lanka’s freshwater landscape, but their CO2 flux dynamics remain poorly characterized and are underrepresented in carbon databases. This study quantified hourly daytime CO2 fluxes and examined associations between CO2 flux and environmental variables in Thalangama Tank (TG) and Huruluwewa Tank (HW), which are situated in contrasting hydroclimatic zones. An automated floating chamber was deployed monthly in each tank between January 2025 and January 2026 to capture the monsoon seasons, while water quality parameters and hydro-meteorological data were monitored simultaneously. TG, a small and shallow tank located in the Wet Zone, showed predominantly positive daytime CO2 fluxes across the sampled campaigns, and no significant seasonal dynamics were observed. In contrast, HW, a large but shallow Dry Zone tank, showed both daytime CO2 uptake and emission, and daytime CO2 flux varied significantly among seasons. Daytime net emission was observed during the first inter-monsoon and northeast monsoon periods, while CO2 uptake was observed during the southwest monsoon and second inter-monsoon periods. At HW, daytime CO2 flux was strongly associated with the ionic gradient, moderated by strong hydrometeorological forcing. Conversely, CO2 flux in TG was weakly related to the ionic gradient but associated with the metabolic balance and rainfall variation. These contrasting patterns suggest that hydrometeorological and physicochemical conditions are important factors associated with daytime CO2 exchange in tropical irrigation tanks. Also emphasize the potential for significant temporal variability in CO2 flux among tropical artificial freshwater systems.