Two small electric airboats, filled with scientists and a tub of mercury, zigzag across a lake on a spring evening in 2001. “No Fishing” reads a sign on the shore – with good reason. At the back of the boat, the mercury slowly disappears into the lake, evenly distributed over the surface. The boats can be found here every two weeks until the autumn, just like the six years after that. They sail on Lake 658 – part of the Experimental Lake Area in the southwestern Canadian province of Ontario. And in the vicinity of the lake, an occasional sprayer flies, sending mercury down over the woods.
Mercury is a heavy metal that pollutes ecosystems worldwide and affects people’s health. There is a natural global mercury cycle, but humans have added large amounts of mercury from the ground since the mid-1800s. Mercury gas is released into the atmosphere via gold mines, coal-fired power stations and heavy industry, which usually ends up in the sea, rivers, lakes and ground via rainfall. There, mercury is converted to methylmercury by bacteria. Methylmercury is bio-accumulative: it builds up during a lifetime, especially in the muscle tissue of, for example, fish. And it’s biomagnifying: the higher up the food chain, the higher the concentrations of mercury in animals. A predatory fish such as mackerel therefore contains more mercury than plankton, or smaller fish. If people ingest too much mercury through fish, it can be harmful to health – especially for young and unborn children. It hinders the growth of the brain, sometimes with lifelong consequences. It is estimated that 1.8 million European children have too much mercury in their blood.
unanswered question
Mercury emissions fell sharply between 1980 and 2000, but meanwhile annual emissions have not decreased any further. 128 countries did sign the Minamata Convention in 2013, with the aim of reducing emissions. As a result, a further decline is expected in the coming years. One big question, however, remains unanswered: how long will it take for ecosystems to recover?
With that question in mind, Canadian and American researchers sailed across Lake 658. For seven years, they increased the mercury supply to the ecosystem fivefold and analyzed the rate of mercury uptake. They measured the recovery for the next eight years. They used a special variant – or ‘isotope’ – of mercury that has never been emitted by humans before. In this way they were able to find out in measurements which mercury pollution was ‘natural’ and which pollution came from themselves. The researchers recently published a study in Nature† The most important outcome, the authors write: once the supply of mercury to an ecosystem stops, the concentration of methylmercury in fish will quickly decrease.
The amount of mercury added is comparable to the pollution in industrial areas
Yanxu Zhang professor of biogeochemistry
“Mercury permeated the ecosystem incredibly quickly,” says Paul Blanchfield. He is an ecologist and first author of the study. “Within a few weeks we already saw methylmercury in the water and in zooplankton. At the end of the first summer it was found in small fish. After a year, we detected it in larger fish.” Seven years after the first zigzag, at the end of the addition phase, increased mercury levels were seen in every lake resident.
But isn’t it ethically dubious to pollute a lake in the name of science? “When the first studies on this came out ten years ago, I thought the same thing,” says Yanxu Zhang. He is professor of biogeochemistry at Nanjing University, and focuses on the behavior of polluting chemicals in the environment. “But it’s not all that bad. The amount of mercury added is not much. It is comparable to the pollution in more densely populated and industrial areas. In addition, the lake is very remote, so there is no danger to people.” Blanchfield agrees, adding: “The concentrations in the lake have no harmful effects on the fish in the lake. We have also collaborated with local and national governments. The importance of the research had to be big enough, and that was it.”
In addition, mercury seems to be disappearing quickly from the lake. After the mercury supply was stopped, the mercury concentration in the water dropped by 80 percent within three years. Some time later, the mercury levels in fish or other life in the lake decreased. This decrease varied per animal species, mainly because the mercury content does not seem to decrease in individual fish, but only in new generations. In Houtingen – a fish from the salmon family – mercury levels decreased twice more slowly over eight years than in bass, despite bass having much more mercury on average. The main difference between the two: bass live an average of three years, the houting seventeen.
Global Cycle
“The study comes at a good time as we expect a drop in emissions in the near future,” Zhang said. “However, it is still unclear how ecosystems will respond to this. The results of the study are encouraging for the effectiveness of the Minamata Convention.”
The study also fits in with a trend within research into global mercury pollution. Many researchers are trying to model the global mercury cycle — the way mercury spreads around the world through air, seas and soil. Only recently have researchers begun to model how ecosystems respond to changes in the mercury cycle. Zhang was the first a year ago: the model that he in Nature Communications published, predicts effects on mercury in ecosystems and health damage in humans, among other things. “We still need a lot of observations to evaluate the precision of our model,” says Zhang. “I expect this data to significantly improve our model. The study is very relevant for global models.”
Jeroen Sonke, biogeochemist at the National Center for Scientific Research in France, is already looking forward to further research in the coming decades. “Only a quarter of the mercury supply to the lake comes directly from rain, the rest comes from rainwater that enters the lake through the surrounding forest. It is currently unknown how quickly mercury from the forest floor will end up in the lake. The movement of mercury from the forests to the lake could take ten, a hundred or a thousand years.”
“We will continue to observe the lake and the fish to understand how the populations continue to respond to the mercury we have added,” said Blanchfield. “But the small amount of mercury that entered the lake from forests also quickly disappeared from the fish. We are still curious whether the mercury in the forests will be transported to the lake, in order to get a complete picture of what is happening around the lake.”