Researchers have attributed an unexpected greenhouse gas effect in lakes to increased oxygen depletion at the bottom of the water bodies.
A study led by the University of Basel and the Université de Montréal examined the direct and indirect effects of climate warming. The results have been published in the journal Limnology and Oceanography Letters.
The researchers said the main focus of their investigations was the water temperature and greenhouse gas production in the deeper parts of the lakes, which play an important role in the global carbon cycle by acting as large natural bioreactors.
The temperature of a lake represents an important constraint on the amount of carbon dioxide and methane it emits into the atmosphere.
Previously it was assumed that global warming stimulates microbial respiratory processes and the production of these greenhouse gases, while at the same time reducing the carbon storage in lake sediments. But the new research has examined these interactions more closely and discovered unexpected effects, according to a University of Basel release.
"We don't want to question the fundamentals of thermodynamics. There is no doubt that the rates of respiratory metabolic processes in lakes are generally higher at increased water temperatures," said Professor Moritz Lehmann from the Department of Environmental Sciences at the University of Basel. "However, climate change will not cause every lake to warm up everywhere."
Lakes worldwide are not only warming at the surface but they are also losing transparency due to increased algae production and enhanced turbidity of the lake water.
"The surface-water warming and the loss of transparency have the effect that more heat is trapped in the upper layers of the lakes, leaving the deeper waters thermally isolated," said lead author of the study, Dr. Maciej Bartosiewicz from the Department of Environmental Sciences at the University of Basel. "Under some circumstances, this can even lead to the cooling of water masses near the bottom of the lake."
This slows down the respiratory decay processes and carbon dioxide production in the lakes, increasing carbon burial within the sediments.
Model simulations suggest that the observed effects are most pertinent to relatively small and shallow lakes, which make up approximately half of the global lake surface.
"All in all, global warming increases the greenhouse gas potential of lakes, as expected. However, this has less to do with the warming directly, and more to do with increased oxygen depletion at the bottom of these lakes," Bartosiewicz said.