Towards a better understanding of reservoir carbon cycling: drawdown emissions and sequestration in sediments of Chapéu D’uvas reservoir

Abstract

Man-made reservoirs are environments of intense organic carbon (OC) processing. The trapping of high fluvial sediment loads combined to often anoxic bottom waters lead to an efficient preservation of OC in reservoir sediments. At the same time that reservoirs bury large amounts of OC they are significant sources of the greenhouse gases CO2 or CH4 to the atmosphere. Accurate measurements of OC burial in reservoirs are still very scarce, as well as measurement of some emission pathways, for example, the carbon release from sediment periodically exposed to the atmosphere (drawdown zones emission). Moreover, many of the current estimates available lack spatial and temporal resolution. In this study we aimed to evaluate the OC burial in the sediment and the CO2 emissions in the drawdown area of the tropical reservoir of Chapéu D’Uvas (CDU), located in southeastern Brazil. The approaches adopted here include both spatial and temporal variability. The variation in sediment accumulation in the CDU reservoir (from zero to 2.8 cm y1 ) explained 70% or the variability in OC burial rate. Average spatially-resolved OC burial rate since reservoir flooding was 35 g C m-2 y -1 , which is similar to the rates found for other tropical reservoirs. Our results suggest that soil erosion in the catchment contribute with a high share of the reservoir sediment load. The CO2 emissions in the drawdown area of CDU were higher in dry periods (mean = 1163 mg C m-2 y -1 ) compared to rainy periods (526 mg C m-2 y -1 ). In addition, dry exposed sediments had higher emissions (1316 mg C m-2 y -1 ) then wet sediments (685 mg C m-2 y -1 ), although sediment re-wetting by rain events was shown to cause increase in emissions. Drawdown emissions were also significantly higher at night than during the day, implying that usual day measurements may underestimate the actual emissions. We finish the study by presenting a carbon balance for the CDU reservoir, which combines the fluxes measured in this study with values from the literature of water surface diffusive and ebullitive emissions. The burial of organic carbon in the CDU reservoir represented ~30% of the total emissions (436 t C yr-1 ) or, only 17% of total emissions in CO2-equivalent, i.e. considering the global warming potential of CH4.