Lake helps scientists measure ecological health

Little Rock Lake, the site of a landmark study on the effects of acid rain, continues to yield important clues about the long-term health of North America’s inland lakes.

For the past two decades, University of Wisconsin–Madison researchers conducted a famous study on this hourglass-shaped lake, acidifying one-half the lake while keeping the other basin in its natural state. Last year, the northern Wisconsin lake came full circle, returning to its natural condition after its pH levels were dramatically altered beginning in 1984.

The scientists, working at UW–Madison’s Trout Lake Station in Vilas County, discovered that the chemistry of the lake corrected itself naturally, but the biological changes took much longer to bounce back. The work is helping demonstrate the natural resilience of lakes to recover from disturbances like pollution.

While that study is complete, two UW–Madison research projects are producing information on other ecological fronts. They include studies of mercury concentration in lakes and the effects of altering lake “littoral zones” – the sensitive shallow areas near the lakeshore.

This summer, scientists with the UW–Madison Center for Limnology and the Wisconsin Department of Natural Resources are seeking an extension of state legislation that makes this work possible. The legislation restricts public access to the small lake and allows researchers to keep in place a mesh barrier that essentially separates the two lake basins.

Tim Kratz, associate director of Trout Lake Station, says the lake’s unusual shape makes it perfectly suited to be a “sentinel lake,” giving scientists clear ways to measure the effects of pollution, development and other changes.

“Without a sentinel lake like Little Rock, which gives us long-term and high-quality data, it’s hard to draw valid conclusions about how disturbances like acid rain and mercury affect our lakes,” Kratz says.

The mercury project is monitoring the effects of a general decrease in the atmospheric deposition of mercury, likely stemming from less use of high-sulfur coal. It is helping reveal one of the successful outcomes of the Clean Air Act. Since 1988, in fact, mercury levels in Little Rock Lake have declined between 40 and 50 percent.

Little Rock’s reference basin will allow scientists to study mercury accumulation and cycling over a much longer period. Kratz says that opening the lake to fishing now would hinder their study of how changes in mercury concentrations accumulate in fish. A sudden increase in angling pressure would likely affect the growth rates of fish in the lake, he says, making it harder to interpret the effect on mercury on fish growth.

The second project is dealing with the physical structure of lake littoral zones, the shallow outer rim of lakes that are important to fish reproduction and water quality. These areas are especially sensitive to human development, since lakes with highly developed shorelines are often cleansed of dead trees and other natural debris that is vital to the food chain.

UW–Madison researchers would like to use Little Rock Lake to answer questions about what happens to lakes with dramatically altered littoral zones. Kratz says there already is evidence showing that bluegills grow much more slowly in developed lakes that have been stripped of woody shoreline debris.

Kratz says that Little Rock is a rare and nationally important site for the new science of “biocomplexity,” which seeks to understand how living things at all levels interact with their environment. UW–Madison received a $3 million grant from the National Science Foundation in fall 2000 to study how human use of lakes affects the overall health of the ecosystem.

The acid rain project – which took an act of the state Legislature to enable back in the early 1980s – will be a useful watermark for the country as acid rain levels slowly begin to improve. In the northeast U.S., where lakes were most vulnerable to acid rain, the average pH of rainfall went from about 4.0 at its worst points in the 1980s to around 4.8 today. The improvements are in large part thanks to the Clean Air Act’s capping of sulfur dioxide emissions.

The Little Rock projects are supported by grants from the U.S. Environmental Protection Agency, the NSF and the Wisconsin DNR. They build on a long tradition of whole lake experiments carried out at UW–Madison’s northwoods research outpost.