New mapping of the carbon sink in lakes and reservoirs
Of all the organic carbon leaching from terrestrial ecosystems to rivers and lakes, only about half reaches the sea. The other half is either transformed in inland waters to the greenhouse gases carbon dioxide and methane, and emitted to the atmosphere, or it is stored and buried in the sediments at the bottom of lakes and reservoirs.
While greenhouse gas emission from inland waters has been studied extensively, carbon burial in sediments has received far less attention, even though it represents a long-term carbon sink. Until now, the available estimates of global lake and reservoir sediment carbon burial were rather back-of-the-envelope in nature, and not very reliable.
This study by Raquel Mendonça and colleagues, published in Nature Communications, is based on a collection of published data on sediment carbon burial in 403 lakes and reservoirs across the world, and the development of predictive models from those data. It estimates that lakes and reservoirs bury on average 0.15 billion tonnes of carbon each year, and also quantifies the uncertainty of this estimate to 0.06-0.25 billion tonnes. These numbers are small compared to most previous estimates, but far more robust. The study further quantifies burial for all of the world’s large river basins separately, and finds that particularly high burial takes place in the warm and dry regions of the Earth.
The study also shows that burial is very sensitive to human activities. About 40% of the carbon burial takes place in man-made reservoirs, which means that burial is very likely going to increase given the current global boom in dam construction. Also, intensive agriculture increases burial through increased soil erosion and eutrophication.
While this study represents a great advance in the global-scale understanding of inland water carbon burial, it also illustrates the scarcity of measurements. More research on the ground is needed, particularly in the tropics, in reservoirs, and in agricultural areas.
This study is a result of the Hydrocarb project, funded by the European Research Council (ERC) to investigate the carbon balance of tropical reservoirs.
Original article: Mendonça, R., R. A. Müller, D. Clow, C. Verpoorter, P. Raymond, L. J. Tranvik, and S. Sobek (2017), Organic carbon burial in global lakes and reservoirs, Nature Communications, 8, 1694, doi:10.1038/s41467-017-01789-6.