How toxic mercury moves through the environment—and accumulates in the fish that people eat—has been known for decades. Now, scientists have discovered an unexpected way that the neurotoxin circulates in lakes, hitching a late-night ride inside small predatory crustaceans dubbed “ghost fleas.” The finding helps explain why some lake fish contain surprising amounts of mercury. It also suggests researchers who sample lakes only during the day might be missing important clues to how those ecosystems work.
“It’s a cool food web story,” says Celia Chen, an aquatic ecologist at Dartmouth College who was not involved in the research. “This idea that mercury would migrate up—it’s novel.”
Most mercury pollution comes from small-scale gold refining and coal burning. It rises into the atmosphere, circulates globally and then falls again in rain and snow. When mercury reaches low-oxygen environments, such as wetlands and lake beds, bacteria convert it to a toxic form called methylmercury that can accumulate in plants and animals. In humans, exposure results in the highest risk to fetuses and young children, who can experience developmental problems.
Top predators such as tuna concentrate methylmercury in their tissues—hence public health warnings to limit or avoid eating certain fish. But in lakes with lots of algae and zooplankton, levels in fish are generally lower; with more creatures at the base of the food web, the amount of mercury in the ecosystem is diluted, and fish get less. There are puzzling exceptions, however. Fish in lakes on North American prairies, for example, have high levels of mercury, despite the presence of lots of algae and other aquatic life. “That was the mystery,” says Britt Hall, a biogeochemist at the University of Regina, who led the new research. “There was no mechanism to explain it.”
In 1997, Hall’s colleague—University of Regina ecologist Peter Leavitt—measured the mercury in various fish species and zooplankton in Katepwa Lake in the Canadian province Saskatchewan. He and co-workers found that yellow perch that hunted at night contained more mercury than yellow perch that fed during the day. Mercury concentrations also varied among the species of zooplankton, tiny invertebrates that drift around the lake. But the researchers couldn’t fit these puzzle pieces together until years later, when University of Regina postdoc Richard Vogt looked closely at one of the zooplankton, called Leptodora.
Leptodora is a large relative of water fleas, about 1.5 centimeters long with a single enormous eye. (Leavitt calls them “ghost fleas,” because they are also translucent.) In 2013, Vogt and colleagues showed that adult Leptodora, unlike other zooplankton in the lake, migrate up and down daily. During the day, they hide from predatory fish by moving to the bottom of the lake, where there is no oxygen. They swim to the top at night, when most fish aren’t active or can’t see well, to feed on other zooplankton. Vogt also conducted experiments with fish and found that at least one kind—yellow perch—can catch Leptodora in the dark, likely by sensing their vibrations while swimming.
The concentration of mercury in Leptodora is about twice that of other zooplankton in the lake, they report in Environmental Science & Technology Letters. That’s likely because Leptodora eat bacteria or midges that live in the mercury-laden mud. At night, they act like mercury “elevators,” bringing the toxin up from the depths. The proof is in the perch: Yellow perch caught at night contained about twice as much mercury has yellow perch caught during the day, the team writes.
That elevator effect results in “astonishingly different exposures” in fish, depending on whether they feed at night or during the day, says Roxanne Razavi, an environmental toxicologist at the State University of New York College of Environmental Science and Forestry, who was not involved in the research. “Scientists could be missing an important vector of [methylmercury] by predominantly sampling in the daytime.”
Chen says this type of ecological research could also help improve the accuracy of large-scale pollution monitoring. For example, some researchers study changes in the amount of airborne mercury deposited in various lakes by monitoring concentrations in fish; the presence of Leptodora might skew comparisons. “These ecological factors change the amount of mercury that ends up in a standard species,” she says. “It’s not a trivial thing to monitor.”
Hall, Leavitt, and colleagues are moving on to another type of pollution: greenhouse gases. They are now looking at how prairie lakes capture carbon dioxide and whether Leptodora might also transport methane—another potent greenhouse gas—to the surface. If so, the mercury elevator might carry another kind of hazard.