Hotspots of diffusive CO2 and CH4 emission shift seasonally in four tropical reservoirs

Abstract:

Reservoirs are globally significant sources of carbon dioxide (CO₂) and methane (CH₄) to the atmosphere. The patterns of spatial and temporal variability of CO₂ and CH₄ emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing. 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. The diffusive emission of CO₂ was higher during the dry season than during the rainy season, while there was no consistent seasonal patterns for the diffusive emission of CH₄. Our results reveal that not only the magnitude, but also the spatial patterns of diffusive CO₂ and CH₄ flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Additionally, spatial patterns of between-season variability were not consistent for CO₂ and CH₄: areas close to the dam generally showed low seasonal variability of diffusive CH₄ flux but high variability of diffusive CO₂ flux. Overall, we found that reservoir areas exhibiting highest emission rates (‘hotspots’) shifted substantially across seasons. Variance-based sensitivity analysis revealed that most of the variability in diffusive CO₂ and CH₄ flux was primarily driven by short-term variability in k, rather than by gas concentration changes in the surface water. Estimates of total diffusive carbon emission (CO₂+CH₄) from the reservoir surfaces differed by a factor of 2.7 (median; range: 1.1 – 13.0) between seasons, 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.