Spatial and seasonal variability of diffusive carbon flux in contrasting reservoirs: magnitudes and contribution of the aquatic surface and exposed sediment

Abstract

Reservoirs are globally significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. The patterns of spatial and temporal variability in CO2 and CH4 emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing rapidly. We performed spatially resolved measurements of dissolved CO2 and CH4 surface water concentrations and their gas-exchange coefficients (k) to compute diffusive carbon flux from four contrasting tropical reservoirs across Brazil during different hydrological seasons. Diffusive CO2 emissions were higher during the dry season than during the rainy season, whereas there were no consistent seasonal patterns for diffusive CH4 emissions. Our results reveal that the magnitude and the spatial within-reservoir patterns of diffusive CO2 and CH4 flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Areas close to the dam generally showed low seasonal variability in diffusive CH4 flux but high variability in diffusive CO2 flux. Overall, we found that reservoir areas exhibiting highest emission rates (‘hotspots’) shifted substantially across hydrological seasons. Estimates of total diffusive carbon emission from the reservoir surfaces differed between hydrological seasons by a factor up to 7 in Chapéu D’Úvas reservoir, up to 13 in Curuá-Una reservoir, up to 4 in Furnas reservoir, and up to 1.8 in Funil reservoir, indicating that spatially-resolved measurements of gas concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission.