Mercury is a natural element that exists in the air, water, and soil. Exposure to even small amounts of it may cause serious health problems on the nervous, digestive and immune systems, as well as on the lungs, kidneys, skin and eyes. It can also affect the development of the child in the uterus and early in life.
The World Health Organization considers mercury as one of the top ten chemicals of major public health concern. People are mostly exposed to methylmercury, an organic compound, by eating fish and shellfish contaminated by the compound.
As to how mercury moves through the environment and accumulates in the fish has been known for decades. But a recent discovery by scientists reveals an unexpected way these toxic compounds circulates in the lakes, by hitching a late-night ride inside the "ghost fleas."
High Concentrations of Mercury on Lakes
The new finding explains why some lake fish contain surprising levels of mercury. It also suggests a food web story that happens during the night that scientists might discover as they only sample lakes during the day.
Celia Chen, an aquatic ecologist at Dartmouth College who was not involved in the research, remarks that the idea of mercury migrating up the water surface is something new.
Most mercury pollution came from small-scale gold refining and coal-burning facilities. It evaporates to the air and circulates globally then falls again as rain and snow. When it drops to low-oxygen environments such as wetlands and lake beds, the bacteria will convert them into methylmercury that can accumulate in plants and animals.
Public health warnings limit or avoid eating certain fish, such as tuna since they contain certain amounts of methylmercury. But in lakes, levels of mercury in fish are generally lower; with more creatures below the food chain, the amount of mercury in the ecosystem is diluted, and fish only gets less.
However, there could be an exception to that such as the fish in lakes on North American prairies which have high levels of mercury despite the abundance of algae and other aquatic life, ScienceMag reports.
"That was the mystery," says lead author Britt Hall, a biogeochemist at the University of Regina. "There was no mechanism to explain it."
In 1997, ecologist Peter Leavitt of the University of Regina and Hall's colleague measured the mercury levels of fish species and zooplankton in Katepwa Lake in Saskatchewan, Canada. He and co-workers found that the yellow perch who hunt during the night has higher levels of mercury compared to the morning perch.
Furthermore, species of zooplankton, tiny invertebrates that drift around the lake have varied mercury concentrations. Researchers could not find the link between these puzzle pieces not until years later when postdoc Richard Vogt of the University of Regina took a closer look at one of the zooplanktons, called Leptodora.
Ghost fleas: Late-night Mercury Elevators
Leptodora is a large water flea, about 1.5 centimeters long with one enormous eye. Leavitt calls them
"ghost fleas" because they are translucent. These species migrate up and down the lake every day to hide from predatory fish and move to the bottom of the lake where there is no oxygen.
Leptodora contains twice the amount of mercury that other zooplankton. Scientists believe that the bacteria or midges that the ghost fleas eat on the mud are rich in mercury, hence the high concentration found in them.
At night, ghost fleas act as mercury "elevators," bringing the toxin up from the mercury-laden mud below. Vogt concluded that the yellow perch at night could catch Leptodora in the dark, by sensing their vibrations while swimming.