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Northern Wisconsin's pristine waters are valued for providing recreational enjoyment for people and critical habitat for wild species. For example, the Northern Highland/American Legion State Forest (NHAL) in Vilas, Oneida and Iron counties has more than 900 lakes and 300 miles of streams within the 225,000-acre forest. Many of these lakes are protected from the pressures development can bring, but they are still subject to other environmental stresses. About 25 years ago consequences from the long-range drifting of atmospheric pollutants raised concerns about acid rain, soon followed by concerns about mercury deposition and climate change.
Long-term research on one NHAL lake continues to provide insight into the consequences of atmospheric pollution. Research on Little Rock Lake began in 1983, three years before Wisconsin's landmark legislation on acid rain was signed into law by then-Governor Tony Earl. The research has continued for 24 years, providing the longest record of environmental responses to acid rain, mercury rain and climate change for any lake in the world. We'd like to share some of the lessons learned from this research that suggest ways to preserve the quality of our northern waters.
Little Rock Lake is a small, clear-water lake that sustains a warmwater fishery of yellow perch and largemouth bass. It is located in Vilas County about three miles southwest of the UW Trout Lake Research Station. Like most Vilas County lakes, Little Rock is a seepage lake, where no streams enter or drain the waterway. More than 98 percent of the lake's water comes from rainfall and snowmelt, so Little Rock is highly sensitive to atmospheric pollutants. Its shape also makes it an ideal water to study and simulate how such lakes respond to acid rain: The 45-acre lake naturally forms two lobes with a narrows between the two segments.
The original experimental design was simple but elegant. The two lobes were divided by stretching a flexible, impermeable barrier across the narrows. The plastic dividing curtain had floats on the top and was anchored on the bottom to form an effective barrier. Initially, the divided basins were monitored to ensure that the barrier itself had no effect on water quality or aquatic communities. Then one basin would be gradually acidified using small doses of sulfuric acid to simulate increasing acidic deposition. The other basin (the reference basin) remained untreated as a reference to measure the variable effects of weather.
Water quality, plankton, bottom-dwelling organisms, fish, and the natural biological, geological and chemical cycles would be monitored continuously in both basins as the treated basin was gradually acidified. The experimental effects were then compared to conditions in other lakes where acid rain impacts were suspected. The acidification phase of the experiment was planned to run for six years, after which acidification would stop. Then recovery of the treated basin would be monitored to determine whether lakes would return to their natural state if acid rain abated.
Scientists and students involved in the experiment had to make difficult career decisions. Final results would not be known for at least a decade, and the interim results were highly uncertain. But these concerns were quickly assuaged after the first two years of acid addition, because adding even very small amounts of acid brought about substantial changes to lake chemistry and biology. The lake was much more sensitive to acid rain than anyone had suspected.
Among the more obvious responses was increased water clarity, consistent with observations by scientists in the northeastern U.S., Canada and Sweden who had reported that where acid rain fell, lakes that had previously supported healthy fisheries became clear and fishless. The scientists suspected acid rain might be the culprit. In Little Rock Lake, clearer water allowed dense green algae growth on the lake bottom. Acidification also slowed fish growth rates. By the end of the acidification, largemouth bass were unable to reproduce successfully; eggs were laid, but they failed to hatch. The bass population got older and, on average, the fish got bigger because no young bass were being added to the population. This result was also consistent with observations in other regions where acid deposition was high. For a while, fishing was very good, and then the fish disappeared altogether.
Another early response was increased mercury contamination in fish in the treated area. Mercury investigations were not included in the original design, but supplementary studies showed that mercury concentrations in perch from the treated basin were higher than in perch from the reference basin. This finding led to a comprehensive study of mercury cycling in Little Rock Lake and to potential links between acid rain and mercury contamination in Wisconsin lakes.
The mercury studies required new sampling and analytical methods. Concentrations of mercury and methylmercury (the chemical form that accumulates in aquatic food webs) were too low to be detected by conventional techniques. Sample contamination was a major problem. Scientists needed to wear special "clean" suits that were free of lint and dust in the field and lab. All containers and reagents needed to be scrupulously free of mercury. Highly sensitive analytical techniques needed to be developed as well. It took several years to make these advances, but by the late 1980s the fundamental aspects of the aquatic mercury cycle had been worked out – the first time for any pristine lake in North America.
We learned that rainfall is the principal source of mercury to northern Wisconsin lakes and their watersheds. After entering lake water, atmospheric mercury escapes back to the atmosphere as a gas, becomes buried in sediments, or is converted to methylmercury by certain bacteria. Methylmercury is passed up the food chain where it poses health risks to animals that eat fish, including humans. Along the chain from water to fish, the concentration of methylmercury can increase 10 million-fold. This phenomenon is called biomagnification, and methylmercury is one of the few toxic substances known to biomagnify in nature.
When sulfuric acid was added to the treated basin, it stimulated the growth of methylating bacteria that inhabit the bottom waters of the lake. These sulfate-reducing bacteria inadvertently produce methylmercury as a by-product of their growth. So during acidification, methylmercury production increased. As the lake de-acidified, these bacteria also declined and methylmercury production decreased again. The fish tipped back and forth between being more contaminated and less contaminated as conditions changed over the course of a few years.
As the treated basin recovered, scientists unexpectedly observed that methylmercury levels declined in the reference basin too. Researchers discovered the reference basin was responding to the effects of cleaner air, as both mercury and acid rain levels have declined substantially over the past 10 to 25 years. The decline in mercury may be due to less commercial and industrial use of mercury in products such as paint, batteries and electrical switches.
Regional reductions in acid rain and mercury rain lowered mercury levels in the water and fish of Little Rock Lake as well as across the board in other northern lakes. However, there is new evidence that the unexpected declines may have suddenly reversed in Little Rock Lake for another unanticipated reason. In the year 2000, scientists were surprised by data that hinted that the lake was becoming more acidic again. The concentration of sulfate in both basins was rising, pH was falling, and the concentration of methylmercury was rising too. Notably, the reversals were occurring despite continued declines in acid rain and mercury rain.
Further monitoring suggests that climate change may be driving the re-acidification of Little Rock Lake and, perhaps, other lakes in the region. Climate change is predicted to have several environmental consequences in northern Wisconsin. In addition to warmer average temperatures, seasonal precipitation patterns may shift, with more precipitation coming in the winter and less in the summer. Less rain in summer, paired with increased evaporation caused by warmer temperatures, could trigger more severe summer droughts and lower water levels in northern Wisconsin lakes.
The reversals observed in Little Rock Lake coincided with an extended period of low water in NHAL lakes. Water levels began to decline in 1998 and remain very low. Studies in Canada document what might be called an "acid drought effect" – a phenomenon whereby sulfate that had been reduced by bacteria is re-oxidized when shallow sediments are exposed to air during drought. Following heavy rain or spring melt, sulfuric acid is regenerated and washes back into the lake. In Little Rock Lake, re-acidification and an increase in methylmercury began about 1999 – one year after the onset of drought conditions.
Future effects of climate change and other human activity remain uncertain for the NHAL lakes. To document these changes, Little Rock Lake has been designated as one of three "sentinel lakes" in the region that will be monitored quarterly to compare their behavior to changes in weather and atmospheric deposition over five- to ten-year periods.
In addition to climatic change, there is growing concern that acid rain and mercury rain levels may increase relatively soon. To meet the anticipated demand for electric power, roughly 150 new coal-burning power plants may be constructed in the United States over the next decade. Several coal-burning facilities are either under construction or planned for Wisconsin and many more in neighboring states. Although new power plants generally employ cleaner technologies than older plants, a net increase in the emission of greenhouse gases, sulfur dioxide and mercury is likely unless older power plants are retired or upgraded.
Research results from Little Rock Lake illustrate that freshwater ecosystems can respond to environmental changes in unexpected but explainable ways. They show that one key to understanding environmental change is long-term monitoring. In the coming years, Wisconsin DNR scientists and their colleagues will continue following the status of Little Rock Lake and the other sentinel lakes of northern Wisconsin.
Research on LRL has been supported by funds from the U.S. EPA, U.S. NSF, EPRI, the Lake Superior Basin Trust, the Potawatomi Community of Forest County, and the WDNR. The acid-rain experiment on Little Rock Lake was undertaken in 1983 to determine how such lakes would likely respond to future increases in acid deposition. The experiment was jointly conceived by scientists in the Wisconsin DNR and the UW-Madison Center for Limnology. They reached out to researchers from other UW campuses, the University of Minnesota and the U.S. Geological Survey to form the primary research team. Over the ensuing 24 years, hundreds of students and scientists from around the world have participated in studies on the lake.
Carl Watras and Ken Morrison are lake researchers with DNR's Science Bureau and the UW-Madison Center for Limnology at the Trout Lake Research Station in Boulder Junction.