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Some scientists studying the Great Lakes say they’ve figured out what is causing mercury levels in the fish to increase or remain steady, despite a reduction in area pollution.
Three years ago, some scientists were puzzled by an increase in mercury levels in areas of the Great Lakes despite a prior steady decline in mercury levels since the 1970s. The levels still weren’t surpassing U.S. Environmental Protection Agency thresholds, according to a 2017 article by the Detroit Free Press, but scientists were stumped by how this was happening.
But recent studies show that there are several connected reasons involving the food web and invasive species, that account for the steadfast mercury in the Great Lakes' fish.
“We are disappointed that the concentrations haven’t dropped more, but it’s not because of our actions, at least in terms of mercury management,” said Ryan Lepak, a post-doctoral researcher for the National Science Foundation. “It’s because of invasive species and ecosystems changes.”
Many fish in the Great Lakes are above what would cause agencies like the Michigan Department of Health and Human Services to issue consumption advisories, Lepak said. But that’s mainly because when people fish in the Great Lakes, they’re going after big fish. And those big fish are the ones that are more likely have high levels of mercury.
“Mercury is unique in the sense that it undergoes what is called biomagnification. Basically saying that you intake what you eat and you have then more of it than the thing you consumed,” Lepak said.
For example, plankton has more mercury than the water it swims in. Whatever eats the plankton is going to get more mercury than the plankton, and so on up the food chain with each fish getting a little more mercury in their system.
Lepak explained that the mercury levels in the lakes themselves actually isn’t very high, in fact they’re among the lowest levels of any ecosystems in the world. But because the Great Lakes are a very cold, hospitable ecosystem, the fish tend to slowly get bigger and live longer.
“But they’re still eating to survive,” Lepak said. “And so they’re eating, they’re still picking up mercury, but they’re growing at a slow rate. So then they get more mercury as well.”
Brandon Armstrong, Aquatic Biology Specialist for EGLE, said the agency is seeing increases in mercury in top predators, like walleye and lake trout, but not in carp. A major culprit for a decrease in pollution not meaning a decrease in fish mercury levels? Invasive species.
The introduction of invasive species to the Great Lakes has changed the food web for the fish that were already there.
“The resource managers have their plates full with this," Lepak said. “We started to get some really great traction on all these mitigation efforts...These invasive species have just altered the entire narrative.”
In Michigan, EGLE has been keeping an eye on zebra mussels. The zebra mussels filter water, gallons of it per day, Armstrong said. While they filter, they pick up contaminants like mercury. Those zebra mussels, now with mercury in their system, are then eaten by goby, which is eaten by larger fish and on up the food chain.
“So, you have a change in that food web,” Armstrong said.
Quagga mussels and zebra mussels changed the way the whole Great Lakes system works, Lepak said. That change in the food web made it more difficult for the fish that were already there to get a meal. That has led to fish working harder to find food that is less nutritious.
“So you can see a scenario where you’re basically eating from your food at a similar or higher rate, getting the same amount of mercury as before, but now you’re not growing as fast because it’s not as good of a meal,” Lepak said. “And actually we see that.”
While it may seem contrary to fishing desires to “catch the big one,” smaller fish are safer to eat. That’s because smaller fish tend to be younger and as a result have had less time to absorb mercury and other contaminants.
Mercury levels in seafood tend to not be problematic for most adults. But too much of it, or exposure for certain populations, can be harmful. Pregnant women or women planning to become pregnant, for example, should avoid mercury.
The U.S. Food and Drug Administration recommends a maximum of two to three servings a week of “best choices” fish, like tilapia, freshwater trout, shrimp, whitefish, salmon, and freshwater or ocean perch. But the agency cautions against fish with the highest mercury levels, like bigeye tuna, orange roughy, and marlin.
No factors can be taken in isolation, Lepak said. It’s a combination of multiple factors playing a role that determines the amount of mercury in the fish, and that includes the human impact.
Despite recent moves by the President Donald Trump’s administration to cut back on pollution regulations, pollution especially from emissions, is down, Lepak said.
But certain areas of the Great Lakes have seen an increase in mercury concentrations over the last 20 years, according to the state Department of Environment, Great Lakes, and Energy (EGLE), though other areas are experiencing either no trend or a decrease in mercury.
Much of the mercury in the Great Lakes that comes from pollution comes from atmospheric emissions, caused by humans, said Armstrong.
“We know that coal fired power plants and waste incineration and some mining and smelting operations contributes to mercury in the atmosphere,” Armstrong said.
This is despite a decline in emissions over the past few decades and some of the lowest concentrations of mercury in waters and sediments that the Great Lakes have ever seen, said Sarah Janssen, research chemist with the U.S. Geological Survey. But certain areas there is no response to the decrease, or even an uptick in mercury levels.
“Things like invasive species, where fish are eating, what they’re eating - it’s all tied together," she said. "So the source is a very important part, but also the ecological aspects of these fish are also important because that dictates how much you can consume of each different fish species, and which ones you can’t.”
As a result, ending pollution just isn’t enough.
“If you turned off all pumps today in the U.S., the fish would not go to zero. That’s not what I’m saying. It’s too complex for that,” Lepak said. “But I do think you would see, I know you would see, an immediate response in the fish here. An improvement to these fisheries.”
While invasive species and pollution are known to have an impact on mercury levels, there’s still a question of whether the large fluctuations in water levels are adding to the problem.
The impact of the Great Lakes' massive changes in water levels over the last few years on mercury levels has not been closely studied. Nevertheless, it could be a contributing factor, said Carl Watras, research scientist at the Wisconsin Department of Natural Resources.
Watras studied the impact of water levels on mercury amounts in inland lakes in the Great Lakes region. By monitoring the changes in mercury levels in fish and loons from hundreds of Wisconsin lakes over the course of 30 years, he found that the water and mercury levels were fluctuating together.
But whether this applies to the fluctuation of mercury levels in fish in the five Great Lakes, is still up in the air, and needs further study, he said.
“I probably wouldn’t put a bet either way on which way it would go,” he said. “It’s probably because the effects of the water level change in those large lakes, in the critical area...it’s smaller than it is on the inland lakes.”
Lepak believes water levels are of small relevance to mercury in Great Lakes fish. The process that creates methylmercury, the mercury that is actually in fish, can have a bigger impact in smaller lakes. Instead, he said the focus really needs to be on the evolving ecosystem and the size of the fish.
“These ecosystems are actually super, squeaky clean for mercury, but all the other factors are against the fish,” he said.
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