If you are of an age that you can remember back to the 1970s, there was a lot of talk about acid rain. While it had been identified as early as the 1850s, increased industry around the globe in the late 20th century made the problem much worse.
Acid rain was identified as having a negative impact on soil, animals, trees, and surface water. In 1980, the US Congress passed an act to encourage scientific research on acid rain, and in 1983, a large-scale experiment was designed right here in Wisconsin. Thirty years later, the Little Rock Lake experiment has yielded a lot of important data. And on July 29, 2013, the project entered a new phase when the curtain dividing the lake was removed.
Tim Kratz  is the Director of the University of Wisconsin Trout Lake Station , which is part of the Center for Limnology  on the UW-Madison campus. His research interests include the long-term, regional ecology of lakes, the interactions between people and lakes, and wetland ecology. He has been involved with the Little Rock Lake research projects since the beginning in 1983. “People were doing surveys of lakes that had various acidity levels, noting that the ones that were more acidic tended to be clearer, fish were struggling to reproduce and so on, but it was unclear what the cause-effect link was,” Kratz explains. So in order to have a control subject as well as an acidified lake, the decision was made to divide a lake in half for purposes of experimentation.
Little Rock Lake was nearly perfect. Kratz says “It’s small enough that we were able to manipulate it. It’s hourglass shaped, and so there was a place where it was relatively straightforward to put a barrier across. He adds “it also had the right chemistry: Susceptible to the effects of acid rain, but not yet affected. … It was an ideal setting.” In addition, the lakeshore was undeveloped, so the human factor was removed from the experiment, which allowed for even more control.
According to Kratz, the curtain was installed in 1984 and that year was used to ensure that the two sides remained similar. Then, in the next year, the process of acidifying half of the lake began. Over several years of gradually acidifying it more and more, the pH was brought down to 4.7. After that, the next phase of the experiment was to monitor the lake’s recovery.
One of the impacts noted on certain fish, like largemouth bass. In the acidic side of the lake, the young fish were unable to grow large enough to survive the winter under the ice. So there were large losses of the higher level predator fish. That was noticed even in the early stages of acidification. At the later stages of acid being added, even the fish eggs were noticeably harmed.
Another effect on the fish was that toxic mercury was stored at higher levels in the fish tissues as the acidification increased. That would clearly impact people who were fishing and eating those fish as well. It turns out that the bacteria that were cleansing the lake of the acid were producing methyl mercury as a by-product. This connection was not seen in the control area of the lake, so it was definitely linked to the acid level.
Carl Watras  is a research scientist with the Wisconsin Department of Natural Resources , as well as a research fellow with UW-Madison's Center for Limnology. He works primarily on lake-atmosphere interactions. Like Kratz, Watras has been involved with the Little Rock Lake research projects since the beginning in 1983. Watras describes that increase in mercury as “unexpected.” He explains that “the bacteria that were feeding on the sulfate and helping to neutralize the acid rain were also producing methyl mercury. We saw large increases in the methyl mercury in the water, and in the fish of that treatment basin relative to the reference basin.”
Kratz explains that having a large scale project span three decades, in such a controlled environment, and with a control set right next to it allowed for an unusual range and depth of research. “One of the nice things about the whole lake acidification is that it’s allowed us to make a real comprehensive set of measurements. So we were able to measure things about the lake physics, the lake chemistry, the lake biology, and try to put all of those things together into one cohesive story.”
Other results included an already clear lake becoming more clear with the addition of the acid. But on the other hand, there was also a very obvious algal mat that grew on the bottom of the lake. This was noted in other acidic lakes as well. The zooplankton species that had been present before the addition of the acid changed completely. These drastic changes were not a direct effect, but according to Kratz were indirect, and “worked through the food web.” For example, some species multiplied noticeably, but it wasn’t because they were particularly suited to the acidic environment. It was because their predators were particularly ill-suited to it.
Kratz adds, “That was one of the take-home messages on the ecological side. Once you start to perturb a system, these perturbations cascade through various parts of the food web, mostly due to these indirect effects.”