Some scientists who study the Great Lakes say they have figured out what causes mercury levels in fish to rise or remain stable, despite a reduction in pollution in the area.
Three years ago, some scientists were puzzled by an increase in mercury levels in the Great Lakes regions despite a steady decline in mercury levels since the 1970s. Levels still did not exceed US Agency thresholds environmental protection, according to a 2017 Detroit Free Press article, but scientists were baffled by how it happened.
But recent studies show that there are several food web and invasive species reasons behind the unwavering mercury in Great Lakes fish.
“We are disappointed that the concentrations have not fallen further, but it is not because of our actions, at least in terms of mercury management,” said Ryan Lepak, post-doctoral researcher at the National Science Foundation. . “It’s because of invasive species and changes in ecosystems.”
Many fish in the Great Lakes are above what would prompt 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 preying on the big fish. And these large fish are the ones most likely to have high levels of mercury.
“Mercury is unique in that it undergoes what is called biomagnification. Basically, by saying that you consume what you eat and then you have more of it than what you have consumed, ”Lepak said.
For example, plankton contains more mercury than the water in which it swims. Anything that eats plankton is going to have more mercury than plankton, and so on up the food chain, with each fish getting a little more mercury into their system.
Lepak explained that the mercury levels in the lakes themselves are actually not very high, in fact they are among the lowest levels of any ecosystem in the world. But since the Great Lakes are a very cold and hospitable ecosystem, fish tend to grow slowly and live longer.
“But they still eat to survive,” Lepak said. “And so they eat, they always pick up mercury, but they grow at a slow rate. So they also get more mercury.
Brandon Armstrong, an aquatic biologist for EGLE, said the agency is seeing an increase in mercury in top predators, such as walleye and lake trout, but not in carp. A major culprit for a decrease in pollution that does not mean a decrease in mercury levels in fish? Invasive species.
The introduction of invasive species into the Great Lakes has altered the food web of fish already there.
“Resource managers have their plate full with this,” said Lepak. “We’ve started to get really good traction on all of these mitigation efforts … These invasive species just changed the whole narrative.”
In Michigan, EGLE monitors zebra mussels. Zebra mussels filter water, gallons per day, Armstrong said. As they filter, they pick up contaminants like mercury. These zebra mussels, now with mercury in their system, are then eaten by the goby, which is eaten by larger fish and up the food chain.
“So you have a change in that food web,” Armstrong said.
Quagga mussels and zebra mussels have changed how the entire Great Lakes system works, Lepak said. This change in the food web made it more difficult for the fish that were already there to feed. This made the fish work harder to find less nutritious food.
“So you can see a scenario where you are essentially eating your food at a similar or higher rate, getting the same amount of mercury as before, but now you are not growing as fast because it is not as good.” meals, ”Lepak said. “And in fact, we see it.”
While it may seem contrary to fishing desire to “catch the big one”, small fish are safer to eat. This is because the small fish tend to be younger and therefore have had less time to absorb mercury and other contaminants.
Mercury levels in seafood tend not to be a problem for most adults. But too much, or exposure for some populations, can be harmful. Pregnant women or women who are planning to become pregnant, for example, should avoid mercury.
The The United States Food and Drug Administration recommends a maximum of two to three servings per week of the “best choices” of fish, such as tilapia, freshwater trout, shrimp, whitefish, salmon, and freshwater or saltwater perch. agency warns of fish containing the highest levels of mercury, such as bigeye tuna, orange roughy and marlin.
No factor can be taken in isolation, said Lepak. It is a combination of several factors that play a role in determining the amount of mercury in fish, and that includes human impact.
Despite recent actions by President Donald Trump The administration to reduce pollution regulations, especially pollution from emissions, is on the decline, Lepak said.
But some regions of the Great Lakes have experienced a increase in mercury concentrations over the past 20 years, according to the State Department of Environment, Great Lakes and Energy (EGLE), although other regions are not experiencing either a trend or a decrease in mercury.
Much of the mercury in the Great Lakes that comes from pollution comes from air emissions caused by humans, Armstrong said.
“We know that coal-fired power plants and waste incineration and certain mining and smelting operations contribute mercury to the atmosphere,” Armstrong said.
This is despite declining emissions in recent decades and some of the lowest concentrations of mercury in water and sediment the Great Lakes have ever seen, said Sarah Janssen, research chemist with the US Geological Survey. But some areas do not react to the drop or even the rise in mercury levels.
“Things like invasive species, where fish eat, what they eat – it’s all related,” she said. “So the source is a very important part, but the ecological aspects of these fish are also important because it dictates how much you can eat of each different species of fish, and which you cannot.”
As a result, ending pollution is simply not enough.
“If you stopped all the pumps today in the United States, the fish wouldn’t go to zero. This is not what I am saying. It’s too complex for that, ”said Lepak. “But I think you would see, I know you would see, an immediate response in the fish here. An improvement in these fisheries.
Although invasive species and pollution are known to impact mercury levels, the question remains whether large fluctuations in water levels make the problem worse.
The impact of massive changes in Great Lakes water levels in recent years on mercury levels has not been closely studied. Nonetheless, it could be a contributing factor, said Carl Watras, a researcher in the Wisconsin Department of Natural Resources.
Watras studied the impact of water levels on mercury amounts in inland lakes in the Great Lakes region. Monitoring changes in mercury levels in fish and loons in hundreds of Wisconsin lakes over the course of 30 years, he found that water and mercury levels fluctuate together.
But the question of whether this applies to fluctuating mercury levels in fish from the five Great Lakes is still open and needs further study, he said.
“I probably wouldn’t bet one way or the other,” he said. “It’s probably because the effects of the water level change in these large lakes, in the critical zone … it’s smaller than on the inland lakes.”
Lepak believes water levels are of little relevance to mercury in Great Lakes fish. The process that creates methylmercury, the mercury that is actually found in fish, can have a greater impact in small lakes. Instead, he said that you really need to focus on how the ecosystem is changing and the size of the fish.
“These ecosystems are actually great, perfectly clean for the mercury, but all the other factors are against the fish,” he said.
More from MLive: