A new study finds that particularly shallow lakes will emit significantly greater amounts of methane due to increasing eutrophication and climate warming, mostly in the form of bubbles. The study was done in Aarhus University, Denmark, using the longest running freshwater mesocosm climate change experiment in the world to investigate how warming and eutrophication might interact to change methane emissions in the future. Academy research fellow Jari Syväranta participated in the research which was published in Nature Climate Change yesterday.
Methane is a potent greenhouse gas with 25 times greater warming potential than carbon dioxide and is produced for example in the anoxic sediments of lakes. Methane is released from lakes in a number of ways, both by diffusion of dissolved gas and directly by bubbles (ebullition). Shallow lakes are increasingly recognised as playing an important role in global greenhouse gas cycling. Given the number of shallow lakes globally they potentially have a large influence on atmospheric methane concentrations, which continue to rise.
The results of the study were striking as they showed that the combination of increased nutrient loading and warming had a synergistic effect on the ebullition of methane. Both nutrient enrichment and warming alone increased annual methane emissions by around 50%, particularly increasing the proportion of methane released by ebullition. In stark contrast to this, when nutrient levels were high, warming increased total methane emission by at least six fold and in some cases 17 fold, and the proportion of ebullition increased to 95% of total annual methane flux.
Nutrient enrichment, or eutrophication, is the most common human impact on fresh waters, with all lakes in agricultural landscapes likely to be impacted. A feature of eutrophication in shallow lakes is the loss of biodiversity and the replacement of submerged plants by phytoplankton as the dominant primary producer. The current study identified the abundance of submerged plants as a key predictor of methane ebullition. When plants were abundant, methane ebullition was reduced compared to when plants were absent, even at higher temperatures.
Dr. Thomas Davidson, Department of Bioscience, Aarhus University, thd (a) bios.au.dk
Dr. Jari Syväranta, Department of Environmental and Biological Sciences, University of Eastern Finland, jari.syvaranta (a) uef.fi, 0505427680