By comparing clear mountain lakes with brown forest lakes the scientists have been able to show that what controls production in lakes is light. This runs counter to conventional truths in lake research that says that productivity is determined by access to nutrients, such as phosphorus.

“In the brownest lakes sunlight can’t penetrate more than about two meters. In clear mountain lakes, the light can reach down to depths of 15-20 meters and lead to high production of algae on lake bottoms,” says Jan Karlsson, associate professor at Climate Impacts Research Center (CIRC).

The majority of the world’s lakes are small and poor in nutrients, and they contain organic material that was washed into the water from the surrounding land. This organic material colors lakes brown, which makes it difficult for light to reach the bottom. The problem is that the algae that live on the lake bottom need sunlight for their photosynthesis. The algae provide food for various bottom-dwelling animals, which in turn are eaten by fish. Limited light penetration thus has negative consequences for all living beings in a lake.

These findings mean that we can expect climate change to disrupt production in lakes. Higher temperatures and thawing of permafrost, along with shifts in precipitation, can lead to relatively rapid changes in the transport of organic material to lakes. In the long term higher temperatures also entail that vegetation will climb higher up mountainsides. This would lead to greater production and transporting of organic material to lakes.

“The climate impacts lakes, and in the long run we can expect more brown lakes with reduced productivity,” says Jan Karlsson.

The study is a result of collaborative work done by Jan Karlsson at the Climate Impacts Research Center (CIRC) and Pär Byström, Jenny Ask, Per Ask, Lennart Persson, and Mats Jansson at the Department of Ecology and Environmental Sciences, Umeå University, in the robust research environment “Lake Ecosystem Response to Environmental Change (LEREC),” which is funded by the Swedish Research Council Formas.