By Sally Pobojewski
News and Information Services
A new study shows that the world’s lakes have more carbon dioxide than they can use. Instead of soaking up atmospheric CO2, as many scientists believe, lakes are pumping carbon dioxide back into the atmosphere.
“The world’s lakes are sources, rather than sinks, for atmospheric CO2,” reported George W. Kling, assistant professor of biology, in the Sept. 9 issue of Science. “Our study of data from 1,835 lakes found that—whether they were in arctic, temperate or tropical areas—nearly 90 percent of all lakes had carbon dioxide levels that averaged three times greater than the surrounding atmosphere.”
Kling is one of four researchers in the first global study of the gas composition of lakes, which was funded by the National Science Foundation. Co-researchers are Jonathan J. Cole and Nina F. Caraco of the Institute of Ecosystem Studies, and Timothy K. Kratz of the University of Wisconsin, Madison.
The study is significant because it indicates that land plants and soils may play an important role in regulating lake ecosystems, according to Kling. In a 1991 study of 29 arctic lakes, Kling discovered that carbon from decaying plant material on land was the source for high levels of carbon dioxide in the lakes.
Kling believes this land-to-water carbon transfer accounts for CO2 supersaturation levels in all the lakes he and his colleagues studied. “Only a few people have made the connection yet on how important input from terrestrial sources is to a lake’s carbon cycle,” he said.
Although the precise mechanism for the carbon transfer is still unknown, Kling believes it goes something like this: “Plants take up carbon dioxide from the atmosphere during photosynthesis. When plants die, dissolved organic carbon and carbon dioxide from decaying plant tissue enters groundwater, where it washes into lakes. Once in the lake, bacteria degrade the organic matter producing even more carbon dioxide.”
As scientists struggle to predict the impact global warming will have on the Earth’s ecosystems, Kling believes understanding the mechanism of carbon transfer between land and lakes will become more important.
“One thing this study makes clear is that we can’t count on lakes to be a buffer against increasing levels of atmospheric carbon dioxide generated from burning fossil fuels,” Kling said.
“If we want to understand how environments will change in response to future global climate change, we must understand the nature of land-water interactions,” Kling said. “Currently, we know little about the rates or mechanisms of carbon cycling between land and water. What are the controls on the system? Right now, we have no answers.”
Kling emphasized that scientists should not rush to revise their estimates of the global carbon budget based on the results of his study. “Carbon dioxide emanating from temperate and tropical lakes isn’t a new addition to the system. It doesn’t change the total amount of carbon in circulation. It tells us more about what’s happening inside the ‘black box’ of the global carbon cycle.”
