A 'Complete Surprise': New Research Shows Higher Lake Levels Equal More Mercury In Fish

Apr 2, 2020

Dr. Carl Watras, the lead researcher on the project connecting lake levels to mercury levels in fish.
Credit Ben Meyer/WXPR

Are the Northwoods walleye you catch safe to eat, or do they have too much mercury?

The answer is tied to several factors, but new research shows a surprising variable might have the biggest effect.

The water level of the lake where you caught the fish could tell you more about its safety than anything else.

DNR Fisheries Scientist Greg Sass with a walleye from a local lake.
Credit Greg Sass

The realization of the connection started years ago, when lakes researcher Dr. Carl Watras found an interesting trend.

The water level on Northwoods lakes, and even the Great Lakes, followed a pattern like a predictable roller coaster: up and down, up and down.

“When we plotted all of those data, going back to the 1930s, they all oscillated together.  They were dancing to the same beat,” said Watras, a researcher at the UW-Madison Trout Lake Station in Boulder Junction.  “It was about a 13-year cycle.”

Continuing research by Watras and colleagues revealed surface temperatures in the Pacific Ocean directly impacted water level trends in our lakes.

He thought his roller-coaster chart was an interesting piece of research, but might not say much more about lake ecology.

“We didn’t think this was necessarily connected to anything else that’s going on in lakes, except it’s very inconvenient for people to keep taking out docks,” he said.

Researchers found the levels of mercury in walleye tend to oscillate with lake levels.
Credit Carl Watras

In a separate study, Watras’ team was looking at mercury levels in walleye from the early 1980s to about 2015.  The team analyzed data on about 1,000 walleye from 200 lakes in Oneida, Vilas, and Florence counties.

They were hoping to find out whether government-mandated mercury emission reductions from industries were having a positive impact on mercury in fish.

But they graphed their data and saw another roller-coaster, one they had seen before.

“That looked really familiar.  We scrambled and mapped those fish data onto the water-level data.  They matched.  It was a complete surprise,” Watras said.

Mercury levels in fish tracked almost perfectly with lake levels.

“For a natural system, it’s a strong correlation,” he said.

In this graph, the blue line shows lake levels, while the black dots graph the levels of mercury in walleye.
Credit Carl Watras

The explanation for what happens, according to Watras, requires a little biology and a little chemistry.

When lake levels drop, new land near the shore is exposed.

“The forest marches in,” he said.  “Terrestrial plants grow like a garden in that exposed near-shore sediment.”

When lake levels drop, vegetation moves in, as seen on Little Rock Lake in Vilas County.
Credit Jereme Gaeta

Then, when water levels rise again, many of those plants die, and the organic matter starts decomposing.  In that process, a group of microbes that produce methylmercury eventually takes over.

“Methylmercury is the form of mercury, the organic form of mercury, that biomagnifies in food chains and ends up contaminating fish,” said Watras.

A little methylmercury in algae becomes a little more in zooplankton, and a little more in the organisms that eat the zooplankton, and on up the food chain.

Once it gets into fish, the amount of mercury is no longer little.

At high water, it’s often considerable enough to tip the fish over the level the EPA says is safe to eat.

“The lows can sometimes dip below the safe consumption level, so the fish are ‘clean,’ but the highs are typically above the safe consumption level,” Watras said.

Until the last decade or so, the up-and-down pattern of lake levels, and, therefore, mercury levels in fish, had been pretty standard.  In other words, the roller coaster has stayed on the tracks.

But now, lake levels are at an all-time high, not long after they dipped to an all-time low last decade.

Watras wonders if these extremes are something we can start to expect in the era of climate change.  If so, we could be looking at higher highs and lower lows in our lakes, similarly sending the mercury-in-fish roller coaster on a wilder ride.

“What if climate change is changing the oscillation of lake levels that’s going to exacerbate this phenomenon?” Watras asked.

Watras says, like all scientists, he wishes he had a crystal ball.