Hurried by a crisp north wind that whips up whitecaps on Trout Lake, a squadron of Canada geese sweeps its V-formation across a leaden sky.
The summer tourists who flock to this lake-dotted region have headed south as well, leaving behind year-'rounders - and a small group of scientists in a lakeside lab who are trying to unlock the secrets of the lakes.
In the seven decades since Trout Lake Station was founded, the waters of Wisconsin's North Lake District have become vital economic assets to the businesses and communities on their shores. They mirror those of many similar areas in the United States, Canada, Europe, and Russia, where close-knit chips off the old glacial block fell, melted, and formed lake districts.
"We need to understand how lakes work to manage them well," says Timothy Kratz, a researcher at Trout Lake. Factors ranging from sewage to clearing submerged logs from along the shoreline can have a profound effect on lakes' long-term health.
Beyond economic interests, however, lakes also act as sensitive indicators for local, regional, and even global environmental problems - from a fisherman's live-bait leftovers running roughshod over native species to acid rain and global warming.
But those indicators must be correctly interpreted, notes John Magnuson, director of the Center for Limnology at the University of Wisconsin at Madison. That means closely watching and measuring the lakes' processes and inhabitants over long periods of time.
Today, for example, 20 researchers from around the world are gathered here to study lake-icing patterns, specifically how soon lakes ice over in the winter and how early they thaw in the spring, to see if such data can provide a reliable barometer of climate change. Such a global indicator could give scientists another way to track global warming.
Dr. Kratz gives another example of why it's important to learn how to study lakes: During a long-running drought in the late 1980s, he notes, the state Department of Natural Resources noticed that some lakes in the area were becoming more acidic. Officials assumed that efforts to reduce sulfur-dioxide emissions, which contribute to acid rain, were inadequate.
"Now that the drought is over, the lakes are returning to normal," Kratz says, adding: "You can't understand a three- to four-year drought cycle with a two- to three-year study."
At the federal level, "there's a push to assess the health of ecosystems" across the United States, Dr. Magnuson adds, referring to Vice President Al Gore's recent call for an environmental report card by the year 2001. Architects of the effort view sites such as Trout Lake, which also is part of the National Science Foundation's network of long-term ecological research stations, as sources for the basic information on how to interpret the report card.
Based on evolving research here, Magnuson says, his colleagues are working with the notion that grading individual lakes, even randomly chosen, with a checklist and clipboard "is the wrong way to look at problems." Unless one literally knows the lay of the land and how it affects lakes in a lake district, the "pupil" could get the wrong grade and the wrong aid.
'Siblings' not created equal
Kratz steps out of a pickup truck and onto a narrow pine-covered isthmus that separates a pair of lakes. A stone's throw apart, the two bodies are markedly different in chemical and biological terms. They help form Exhibit A in the case for region-wide studies.
With no streams or rivers connecting them, similar vegetation surrounding them, and similar soils and rock formations beneath them, "one would expect the lakes to be similar too," Dr. Kratz says.
Crystal Lake, the smaller of the two, is remarkably clear, with perhaps 13 species of fish inhabiting the near-shore region. Crystal's companion, Big Muskellunge, on the other hand, has murkier water and some 20 species living in its littoral zone. Trout Lake, another neighbor, hosts 25.
When Kratz and his colleagues at the University of Wisconsin and the Wisconsin Department of Natural Resources measured other factors, such as each lake's natural capacity to neutralize acids, they varied as well.
Things became clearer when the team applied concepts well-known to river ecologists to the lakes. As a brook flows downhill to become a stream and then a river, the waterway's biological and chemical richness grows. Each step is seen as a distinctive stage of development, requiring a somewhat different gauge to measure environmental vitality.
Comparing lake locations with terrain, Kratz's team discovered a similar correlation with the lakes they studied, which depend in large measure on ground water to fill them. The higher the lake, the more dependent it was on rainwater and the less able it was to buffer acids, and the fewer species it harbored.
In Wisconsin's Northern Highland Lake District, "high" is a relative term. Crystal Lake is only about five feet above Big Muskie. The difference in height of the water table surrounding Crystal Lake is only about 8 feet above that of Trout Lake, the lowest of the lakes in the study. But the correlation was there to see, nonetheless.
By applying this notion of a "lake continuum," rather than a set of discreet lakes, the team hopes to develop an approach for predicting which lakes might be most vulnerable to acid rain, for example, or which might be expected to have the most diverse fish and plant populations.
Nor are they interested in a tool useful only in Wisconsin. In February, Magnuson and Kratz are planning an international workshop to compare notes with lake researchers from other parts of the world where ebb and flow of glaciers have left lake districts in their wakes.
Lakes as pollution barometers
Leaves crunch underfoot as Kratz leads a visitor to the edge of Little Rock Lake, a short drive from the lab. Until 1990, half the lake - cut off by a rubber curtain stretched across its hour-glass waist - had received regular doses of sulfuric acid. The other half was left in its original state, protected by the curtain, Kratz explains. The idea was to mimic the effects of acid rain and study them on a whole ecosystem.
Well into its 12th year, the experiment now is aimed at watching how the treated half recovers. The basic question, says Tom Frost, associate director of the Center for Limnology, "is how long does it take?"
The tentative answer: If you look only at the water chemistry, fairly quickly. In fact, he says, the recovery rate is almost a mirror image of the original pollution rate. But the biological recovery rate has been much slower.
In 1984, after getting permission from the state government and with money from the US Environmental Protection Agency, researchers at the lake began adding sulfuric acid to the water on one side of the curtain. Once every two years, the scientists increased the acidity level until in 1990, it approximated the amount of acid in Northeastern lakes.
During the first two years, the number of species on the treated side fell off slightly, but the number climbed fairly quickly as more acid was added.
Dr. Frost, who was interested in tracking the effect on zooplankton, tiny organisms that feed on algae and in turn feed fish, noted that the total amount of zooplankton fell.
And he found that, while what was left of the total community continued to perform its eat-and-be-eaten functions, specific roles performed by one set of species before the acid was added were taken up by a different set of species afterward.
And while the water chemistry has recovered in the treated side of Little Rock Lake, "the zooplankton community has not followed the same script," Frost says. Among the questions raised, he says, is whether one should measure recovery by the number of original species that return, or merely whether the roles those species had in the ecosystem are still being performed, even if they're being performed by different species.
Some researchers studying the effects of acid rain in streams have noted that even though sulfur-dioxide emissions have fallen substantially, water chemistry hasn't recovered, Frost says. "I'd argue that even after the chemistry recovers, the biological community will lag behind that."
"We don't understand why the biological community is lagging," Dr. Magnuson adds. "The 'seeds' are only 6 inches away" on the other side of the curtain.
The next step for Trout Lake's lake-management research is to take a harder look at the human factor, Kratz says over a cup of squash soup in the lab's residence quarters.
Take the case of "coarse woody debris," also known as logs in lakes. Homeowners building on lakes usually remove the submerged wood to make the shore safer for swimmers, docks, and boats.
"But trees in the lake are breeding grounds for some species of fish," he notes. "Remove the trees and you change the biological processes in the near-shore areas. What does that do to the distribution of fish? How does that, in turn, affect human valuation of the lake?"
"By adding social scientists to our research projects, we can add humans to the equation in an explicit way," he concludes. "I think we're in for some real surprises."