Concerns about the presence of mercury in fish keeps a lot of people from consuming what comes off the line. And that’s a concern anywhere there is a large body of fresh water, like, say, Lake Michigan.
Over the last decade, a group of scientists set out to discover if new mercury added to a lake would make its way more quickly into the aquatic food chain faster than “existing” mercury – what is released naturally into the atmosphere, by volcanoes or otherwise, and makes its way into watersheds.
The answer was a resounding yes – and it happens much more quickly than previously thought.
But there’s good news, too. The same long-range study finds that mercury levels in the food web can be lowered almost as quickly once that added mercury is filtered out.
New tools to trace mercury
The METALLICUS study is being conducted by Canadian and American scientists in a staged setting in a lake 250 kilometers east of Winnipeg. Early results were published in a recent online issue of Science of the Total Environment.
UW-Madison scientist Jim Hurley says he and the other researchers have used stable isotopes of mercury and new chemical instruments to trace how the mercury they have added to the lake ultimately gets into fish.
“We added mercury in 10 different doses over the course of the open water season on a lake, so it was from late May until some time in early October,” Hurley says. “And we could clearly see that by September, we were starting to see the signal of that added mercury in the fish in detectable amounts.”
But the mercury that is found in fish is in a different form – called methyl mercury - than the inorganic mercury that gets introduced to a system. Microbes are the cause of this conversion.
Hurley says researchers found this transformation happens more quickly with added mercury, compared to the mercury that can be found naturally in a given system.
“The new mercury that we added appears to be more bioactive than that mercury that’s been sitting in the lake for years and years or sitting in the soils, or transported by streams to the watershed,” he says. “We can see it get incorporated more quickly into phytoplankton.”
Those are then consumed by organisms and fish, which are then eaten themselves, and so on, until you have an accumulated amount of mercury in the top predator fish – which then can pose a threat to human health.
And while Hurley says that there’s always been a pool of mercury that’s made its way into watersheds, its presence has grown four times greater than this “natural background levels” since industrialization began.
Ridding lakes of mercury
Of course, if added mercury can so quickly show up in fish, how quickly can it be removed from a watershed ecosystem? So for the past five years, researchers have looked at what happened to the lake and its fish once they stopped adding in mercury.
“Once we stopped adding isotopes we got a fairly immediate response in most of the food web in the lake and that it began to drop,” he says. “So if you mimic our study with saying, ‘Okay, we want to decrease the amount of mercury that comes in, say, from coal-fired powered plants,’ we were seeing a response really quickly in this lake to lowering the amounts of inputs.”
Hurley suggests this is the most important finding of the research.
“If you can decrease the sources, you should see a fairly rapid response in lakes like the one that we studied.”
Hurley, who directs the University of Wisconsin Sea Grant program, worked alongside other UW researchers as well as scientists from the USGS, the University of Maryland, Oak Ridge National Laboratory, the Smithsonian Estuaries Research Center, various Canadian universities, and the Canadian Department of Fisheries and Oceans.