Wisconsin’s Lakes: The Mercury Conundrum
By: Johnathon Brehm
Carl Watras and a team of Wisconsin scientists have discovered an interesting correlation between water levels and mercury contamination in Wisconsin’s Lakes.
While the battle against pollution and fossil fuels has captivated the world in recent years, Wisconsin has been at the forefront of this war for over 50 years. From Wisconsin Senator Gaylord Nelson founding the idea of Earth Day in 1970 to the passing of the nation’s first acid rain legislation in 1986, Wisconsin has been a model for ecological advocacy. Thus, it is no surprise that another environmental revelation has recently originated from the Badger state. A team of scientists from UW-Madison and the Wisconsin Department of Natural Resources has unearthed a surprising correlation between mercury contamination and water levels in Wisconsin’s lakes.
The findings come out of UW-Madison’s Trout Lake Station, an environmental and limnological research center located in Vilas County. Trout Lake Station’s proximity to over 12,000 lakes makes it an asset to research fellows such as Carl Watras, who has spent over 30 years at the location. A particularly remarkable achievement of his was helping publish a scientific paper in 2014 that quelled widespread fear about the condition of Wisconsin’s lakes.
“In the past 70 years, water levels had never been lower than what they were in 2014,” recalls Watras. “People were panicking.” Watras’s team was able to prove that there are regional water level swings that typically last 13 years from high peak to high peak. For example, 2018 was considered a high-water-level peak, meaning that 2005 should have been the last high-water-level peak. However, the actual last high-water-level peak occurred in 1998. This is what sent many people into a panic in 2014: the low water level peak was uncharacteristically low and was lasting an exceptionally long time. Why did this last water level swing last so long? As Watras explains, “The differences between the amounts of evaporation and precipitation can change, and this drives the water level oscillations. The oscillation itself is a climatic signal. It’s like a heartbeat.” It was from learning about these water level oscillations that Watras and another team of scientists were able to find the correlation between water levels and mercury contamination.
Since 1982, the Department of Natural Resources has been tracking the length, weight, and mercury concentrations of walleye so that this data may be used for research purposes. If mercury concentration is plotted against regional water levels, as Watras did, there is a strong correlation. After, Watras and his team had to determine the meaning of this relationship. What they found out was truly astonishing.
“Water level oscillations are driven by the differences between the amounts of evaporation and precipitation. The oscillation itself is a climatic signal. It’s like a heartbeat.”
As water levels recede over time and more land is exposed to the atmosphere, mercury that is present in a lake is deposited on its shores. The newly exposed land eventually becomes home to grasses, shrubs, and trees, which absorb this mercury and store it. When the water returns to its original levels, these plants drown and are decomposed by bacteria that eventually use up all the oxygen in the flooded zone. Those conditions set the stage for another group of very primitive bacteria that thrive in oxygen-free environments. These bacteria take up mercury and convert it to an extremely toxic waste product: methyl mercury. This toxin eventually works its way through the food web and concentrates at the top of the food chain by a process known as biomagnification. This means that the methyl mercury builds up in predators such as walleye and largemouth bass, making these fish ideal for tracking the mercury contamination in lakes.
Now that a correlation has been established between water levels and mercury contamination using walleye data, Watras and his team are beginning to investigate if other animals exhibit a similar trend. “We are looking to see if we get the same pattern with loons as you do with walleye,” explains Watras. Loons have basically the same diet as large walleye, so it would make sense to see the same relationship to water levels. That’s the current hypothesis. He hopes to find out more about mercury contamination and its effects on lakes by using data from multiple animals. With each discovery that Watras and his team make, they continue to uphold Wisconsin’s rich tradition of environmental advocacy.
