Keeping Up with Global Change
From a focus on plant communities to a broader look at global change, LTER research is revealing how the components of our environment interact.
Albert Einstein once said that chance favors the prepared mind. So, too, are LTER scientists uniquely prepared to learn from seemingly chance fluctuations in global climatewhat LTER program head Scott Collins calls "the surprise years."
A good illustration of this can be found among the scores of lakes that make up the NSF-funded North Temperate Lakes (NTL) LTER site in Wisconsin. A member of the network since the LTER program's start in 1980, the NTL site is managed by researchers at the University of Wisconsin at Madison. The NTL LTER includes two field stations: one in the Yahara Lake District of southern Wisconsin and the othercalled the Trout Lake Stationin the state's northern highlands. While the area boasts hundreds of lakes that are amenable to study, the sites' principal investigators have chosen seven to consistently monitor over the long haul.
If researchers investigate only one lake, they don't know whether their findings are unique to that lake, says University of Wisconsin limnologist Timothy Kratz, a principal investigator for the NTL LTER. Studying many lakes exposes patterns and commonalties that are visible only when researchers investigate environmental conditions over a broad region. The seven lakes of the NTL LTER were chosen because of their representative variety in size and location.
The number of lakes, their different sizes (ranging from quarter-acre bogs to 3,500-acre behemoths), and their distribution from lower to higher elevations, allowed Kathy Webster, then a doctoral student, and other NTL researchers in the late 1980s to conduct one of the first and most informative field studies of how lakes respond to drought.
Year in, year out since 1981, NTL researchers have measured the lakes' chemical composition, tracking fluctuations in calcium, magnesium, alkalinity, and other factors. These persistent measurements paid off in the late 1980s, when the upper Midwest was hit by a major drought. "We were able to look at our lakes pre-drought, during the drought, and after the drought," says Kratz. The results were surprising: Although all of the lakes lost water, only those lakes positioned higher in the landscape lost significant amounts of calcium, an essential nutrient for all organisms. The effect was all the more striking because the elevation difference between the highest and lowest study lakes was only about 33 feet.
What could explain the different level of calcium loss? Groundwater, suggests Kratz. All of the lakes in the study are fed by groundwater seeping through the rocky soil. The groundwater carries with it an abundance of critical minerals, including calcium. But the drought caused the groundwater table to fall below the higher lakes, essentially shutting off their mineral supply.
In a prolonged drought, says Kratz, lakes in higher elevations might become calcium deficient, causing a cascade of biotic effects. Animals such as snails and crayfish would be in trouble, since they require calcium to make their shells. In turn, fish that eat snails would find it harder to get enough food. The higher lakes might also become more susceptible than their low-lying counterparts to the effects of acid rain, since the calcium and other minerals from groundwater can counteract the deleterious effects of acid precipitation.
If changes in the world's overall climate result in droughts that become more frequentas some researchers predict with the advent of global warmingthe chemistry of these two types of lakes will start to diverge. Data of the kind gathered at North Temperate Lakes LTER should help both scientists and policymakers predict and cope with the environmental consequences of global climate change.
"We didn't know the particular event of interest would be a drought," Kratz says. "But we had in place a system of measurements that would allow us to analyze the situationwhatever the event was."