Acid rain: lowdown on health of lakes.

Acid rain: Lowdown on health of lakes

A large share of northeastern U.S. lakes may be suffering severe -- and potentially unrecognized -- ecosystem damage from acid rain, a new study indicates. While the most vulnerable species tend to be ones humans consider relatively unimportant--such as leeches, mollusks and insects--they are integral to a lake's overall health. Indeed, according to the new analysis, their dying out not only is a symptom of the ecosystem's decline, but also sets the stage for the loss of more prized species, such as trout, pike and sunfish.

In 1976, David W. Schindler and his colleagues at the Canadian governmenths Freshwater Institute in Winnipeg, Manitoba, initiated an unusual experiment. Over eight years, they systematically added sulfuric acid to a small Canadian lake, dramatically lowering its pH from a nearly neutral 6.8 to a very acidic 5. As the acidification progressed, the researchers carefully monitored its impact on plants and animals in the lake. They found that crustaceans and many phytoplankton disappeared, fish ceased to reproduce and new algae appeared.

Schindler and his colleagues have now correlated these and related data -- from studies comparing species deiversity in normal and acidified lakes -- with chemical assessments for 6,351 U.S. lakes identified in the Environmental Protection Agency's Eastern Lakes Survey as being acid-sensitive (having soft water). Their findings, published in the May ENVIRONMENTAL SCIENCE AND TECHNOLOGY, suggest many of these lakes have already suffered serious biological impoverishment.

For example, the researchers' analysis indicates that mountain lakes within the Adirondacks, Poconos and Catskills may have lost 69 percent of their leeches, 45 percent of their insects (especially mayflies, dragonflies and damselflies), 50 percent of their mollusks (such as clams), 18 percent of their crustaceans (such as crayfish) and 25 to 30 percent of their algae. But those are "median" estimates for these large regions. Highly susceptible lakes within these areas may already have witnessed a complete elimination of their leeches and mollusks, most of their insects and crustaceans, and more than half of their rotifers (drifting plankton) and fish.

In the past, biological assays of acid rain's effects on lakes have focused largely on sport fish. This analysis is the first to predict a region's full range of losses from acid rain, notes Mark D. Mattson at the New York Botanical Garden's Institute of Ecosystem Studies in Millbrook, N.Y. The study's greatest value, suggests University of Toronto zoologist Harold H. Harvey, may be in shifting the focus from fish to the more vulnerable but ecologically important species farther down the food chain.

Because Schindler's group used summer lake-pH values in making its predictions -- and not the much lower spring-snowmelt values -- their assessments may in fact seriously underestimate species losses, Mattson and Harvey point out. "Even just a few days of very low pH might have a greater effect on the lake than a whole summer's worth of moderately low pH," Mattson explains.