Wisconsin Lake Study Shows Persistence Of Acid Rain Effects

The University of Wisconsin-Madison study, spanning two decades, found that while the chemistry of the lake corrected itself naturally - and fairly quickly -- the biological changes took much longer to bounce back.

This year, the northern Wisconsin lake came full circle, returning to its natural condition after its pH levels were dramatically altered beginning in 1984. Scientists separated the hourglass-shaped lake into two basins with a mesh curtain, keeping one side in its natural state while the other was slowly acidified.

From 1984 to 1990, the test basin was taken from an original pH of 6.1 down in two-year intervals to 5.6, 5.2 and 4.7. Then it was allowed to recover without intervention.

It essentially became a tale of two lakes, as the character of the acidified water began to dramatically change, says Thomas Frost, director of the study and site manager of Trout Lake Station in Vilas County. Frost reported his findings this month at the annual meeting of the Ecological Society of America. "We found that the pH levels had a controlling but indirect influence for nearly every biological factor in the lake," says Frost. "The nature of the food web changed completely."

Sport fish in the lake, such as bass and perch, survived the change but the offspring of fish were unable to survive. The zooplankton in the lake, a critical part of the food chain, underwent a complete revolution. Some once-rare zooplankton took over the lake, while once-dominant species almost vanished.

The acidified lake became almost crystal clear in the process, and ultraviolet light penetration increased, he says. Chemical changes helped a long, filamentous algae nicknamed "elephant snot" to spread across the lake bottom.

Mercury in fish increased with acidification, but study of the lake's recovery has shown that mercury deposition has declined recently.

In the end, Frost says, the lake showed a remarkable resilience by returning to its pre-disturbance conditions. But the biological changes lagged behind the chemistry, taking several years longer to reach its previous balance.

"The entire ecosystem of the lake is much more resilient than individual species," he says. "Some species were decimated and others thrived, but the sum-total of life in the lake stayed the same."

The project - which took an act of the state Legislature to enable back in the early 1980s - will be a useful marker 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.

But that good news is tempered by the evidence from Little Rock that lakes recover slowly from biological changes driven by acidity. Frost says other studies of northeast U.S. lakes have found little improvement in pH levels. The study also demonstrated that what was intended as a single, isolated stress, acid rain, actually created other stresses on the health of the ecosystem.

While the sulfuric acid caps have been an environmental success story, Frost says another major contributor to acid rain, nitric acid, is prevalent in the atmosphere and warrants further study.

The study was supported by the U.S. Environmental Protection Agency and the National Science Foundation. It built on a long tradition of whole lake experiments conducted by lake researchers at UW-Madison.