Sources of organic carbon fueling carbon emissions from tropical reservoirs

Abstract: For a sustainable energy supply, it is of importance to be aware about the environmental impacts from the different energy sources. Hydroelectric reservoirs in tropical areas have been found to emit more greenhouse gases (GHG) than reservoirs from boreal regions. Emerging economies such as Brazil, China and India have developed extensive plans for future constructions of hydroelectric reservoirs, and most of them will be built in tropical regions. Methane (CH4) release has been identified as particularly significant, because it has a much stronger greenhouse gas potential than carbon dioxide (CO2). Therefore, there is a need for a better understanding about methane production in hydropower reservoirs, to develop strategies for the mitigation of the potential impact of hydropower on the climate. Previous research has shown that not only organic matter (OM) from impoundment (flooded trees and soil) can be a substantial source for CH4 emissions from reservoirs, but also OM from other sources. Reservoirs can offer favorable conditions for primary production by aquatic plants, both phytoplankton and macrophytes, i.e. water-living vascular plants. The sustained long-term CH4 emission from tropical reservoirs, i.e. the emission after the flooded trees and soils are decomposed, could thus to a large degree be fueled by the production of new organic carbon (OC) by aquatic plants. This project examines the relative importance of autochthonous OC (originating from inside the system, i.e. aquatic plants) and allochthonous OC (originating from outside the system, i.e. from land) as a fuel for CH4 production in tropical reservoirs. The influence on the production of CO2 and CH4 of different amounts, types, and species of autochthonous sources and one allochthonous source from reservoirs around the area of Juiz de Fora, Brazil, were studied experimentally. There was a great variability in the production of CH4 and CO2 due to the source of carbon. The allochthonous contribution to CH4 production was negligible in comparison to the autochthonous sources. However, increasing amounts of autochthonous OC did not result in equal increases in CH4 production. Further, the degradation of the macrophyte P. stratiotes produced about five times more CH4 than that of phytoplankton, even though phytoplankton is generally considered as a more labile source. Also between different macrophyte species, a great variability in the extent of CH4 production was observed. The regulation of nutrient inputs to reservoirs could help to mitigate CH4 emission. Also, regulating the growth of the invasive species P. stratiotes, which had a particularly big potential for high CH4 production, could be useful for reservoir CH4 emission management. In addition, the degradation of macrophytes on land results in less production of CO2-equivalents than in oxygen-poor bottom waters, therefore harvesting of macrophytes could be another way of reservoir emission management.