Subtle Biotic Changes Have Big Environmental Impacts

By changing the composition of fish populations in a lake, scientists have found a switch by which the flow of carbon between lakes and the atmosphere can be turned on, off, or reversed.

The finding, reported by researchers from UW–Madison in this week’s (July 11) edition of the journal Science, is the first to show that only slight rearrangement of an intact ecosystem’s food web can directly influence the atmosphere.

The discovery is important because it demonstrates that single, seemingly subtle changes in ecosystems can have far-reaching consequences, and are capable of disrupting the fundamental biogeochemical processes of the Earth.

Bass, by preying on the minnows that consume algae-grazing zooplankton, effectively increase the flow of carbon from a lake to the atmosphere by freeing zooplankton from their predators. The zooplankton graze the lake's algae to the point where the algae are no longer a force to use the lake's excess carbon which is then expelled to the atmosphere.

“Linkages in ecosystems are both stronger and stranger than we imagined,” said Stephen R. Carpenter, a UW–Madison limnologist who, with fellow limnologists Daniel E. Schindler and James F. Kitchell, authored the report. “Biological processes have powerful feedbacks to processes that are normally thought to be purely physical or chemical in nature.”

While lakes occupy a very small area of the planet’s surface, the discovery that simple biotic change is capable of altering the exchange of carbon between the atmosphere and the Earth’s surface raises questions of global significance, said Carpenter.

“To what extent could fertilization of the oceans and alteration of oceanic food webs affect global carbon cycles? In fact, runoff from land is now enriching coastal oceans to unprecedented levels, and industrial fishing is causing massive changes in marine food webs. So the global experiment is underway,” said Carpenter.

Carbon, an essential nutrient in lakes, typically flows from the land in the form of dead leaves and other organic matter that accumulates and decays underwater. Usually, these processes lead to a surplus of carbon dioxide in lakes. Excess carbon in a lake is released as a gas, carbon dioxide, to the atmosphere.

When there is a deficit of carbon dioxide, however, lakes draw the gas directly from the atmosphere.

Working on an isolated, undeveloped suite of lakes in Michigan’s Upper Peninsula, the Wisconsin scientists were able to manipulate the flow of carbon between an entire, intact ecosystem and the atmosphere by placing either minnows or bass at the apex of the lake food web.

Bass, by preying on the minnows that consume algae-grazing zooplankton, effectively increased the flow of carbon to the atmosphere by freeing zooplankton from their predators. The booming zooplankton populations grazed the algae to the point where they were no longer a force to use the lake’s excess carbon. The lakes, in effect, became pumps, expelling unused carbon to the atmosphere.

In lakes dominated by minnows, whose menus include algae-eating zooplankton, burgeoning algae populations and their photosynthetic requirements resulted in a carbon deficit, and the lakes become carbon sinks, drawing carbon directly from the atmosphere.

“This effect of fishes on gas exchange results from the changes in aquatic food webs that are regulated by the species of fish present in a particular lake,” said Schindler.

The changes in lakes, Schindler emphasized, will not have implications for global climate. However, the new understanding of the processes that alter the exchange of carbon dioxide between lakes and the atmosphere can be generalized to other ecosystems such as oceans.

“Although the consequences … are much less known for marine systems than for lakes, we should expect that the ecological responses to exploitation are similar in many ways,” Schindler said.

The work done by the Wisconsin scientists was funded by the National Science Foundation and conducted under the auspices of the UW-Madison Center for Limnology.

CONTACT: Stephen R. Carpenter, (608) 262-8690, srcarpen@facstaff.wisc.edu