Hotspots of diffusive CO2 and CH4 emission from tropical reservoirs shift through time

• 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. 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. We used an online equilibration system to measure dissolved CO2 and CH4 concentrations; we estimated k from floating chamber deployments in conjunction with discrete CO2 and CH4 water concentration measurements. Diffusive CO2 emissions were higher during dry season than during 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 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, up to 13 in Curuá-Una, up to 4 in Furnas, and up to 1.8 in Funil, indicating that spatially-resolved measurements of CO2 and CH4 concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission.

• Reservoirs are key for flood control, water supply, and hydropower generation. However, reservoirs are usually not carbon neutral. Studies worldwide point to reservoirs as important net sources of anthropogenic carbon emission to the atmosphere. Carbon emission from reservoirs derives from the decomposition of organic matter. Although carbon emission from reservoirs has been increasingly studied over the past two decades, most studies do not sufficiently describe emissions across space and time. Our study applies highly-resolved spatial coverage of dissolved surface water concentrations and gas-exchange coefficients of CO2 and CH4 to compute rates of CO2 and CH4 diffusion to the atmosphere across distinct hydrological seasons in four contrasting tropical reservoirs. We found that emissions varied substantially over both space and time. More specifically, we found that reservoir areas exhibiting highest emission rates ('hotspots') shifted substantially between dry and rainy seasons. Overlooking the spatial within-reservoir variability across seasons may result in serious under- or overestimations of total diffusive carbon emission from reservoirs, depending on the time and space that studies focus their sampling on. Our work may support scientists in adopting more comprehensive sampling strategies relevant for better constrained upscaling, and, consequently, support informed policy decisions and management actions.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Klimatforskning (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Climate Research (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Multidisciplinär geovetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geosciences, Multidisciplinary (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Oceanografi, hydrologi och vattenresurser (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Oceanography, Hydrology and Water Resources (hsv//eng)

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