The 'devil flea' brings mercury into the human food chain

New research shows that crustaceans throughout Leptodora at night act as an elevator transporting mercury from the lake floor to the surface.

How toxic mercury from the environment accumulates inside fish-like organisms and enters the human food chain has been a topic of concern for decades. In a study published recently in the journal Environmental Science & Technology Letters , biologists from the University of Regina of Canada have discovered a new way of mercury moving inside lakes, related. to a crustacean known as the devil flea.

Leptodora absorbs toxic mercury from the bottom of the lake and carries it to the surface. (Photo: Blickwinkel / Alamy).

Most mercury pollution comes from coal burning and small-scale gold operations. This process causes mercury to evaporate into the atmosphere and circulate globally, then fall to the ground with raindrops or snow.

When mercury enters an environment with low oxygen levels, such as the bottom of a lake or wetlands, bacteria convert it into another toxic compound called methylmercury or methy mercury , which can accumulate. inside animals or plants, especially those at the bottom of the food web.

Therefore, in lakes with algae and plankton, the methylmercury content inside the ecosystem is often low. However, there are exceptions that are difficult to explain as in lakes on the steppe of North America, where some fish have a very high level of mercury accumulation despite their habitats with many algae and small organisms.

To find out, biologist Britt Hall from the University of Regina led a team that measured mercury content in a range of fish and plankton living inside Katepwa Lake in Saskatchewan Province, Canada. . They found that nocturnal predators like yellow perch have significantly higher levels of mercury accumulation than daytime predators. The cause is related to a relative of the water flea whose scientific name is Leptodora.

These transparent creatures about 1.5 cm long are active at night. During the day, they hide from predatory fish by moving to the bottom of the lake where there is no oxygen. Here, they eat bacteria or cast larvae filled with methylmercury, making the body's mercury concentration twice as high as other crustaceans and plankton.

At night, when most fish are no longer active or have no good vision, Leptodora swims upwards to hunt plankton. Therefore, they act as an elevator transporting harmful mercury from the bottom of the lake to the surface. When nocturnal predators like yellow perch eat Leptodora, mercury is transferred to their bodies and can enter the human food chain.

The new findings by Hall and colleagues will help improve the accuracy of large-scale mercury pollution monitoring, as previous studies focused on lake fish. The result may be misleading in the presence of Leptodora.