The Eurasian beaver was nearly extinct by the dawn of the 20^th century, and conservation efforts and new laws regulating the trade of furs have ushered in a new era of beaver productivity across the continent. According to a press release from the American Chemical Society, however, bringing back the beavers may have led to some unintended consequences for modern-day Europe.

In a recent report in the journal Environmental Science & Technology, scientists revealed that beavers setting up new dams and colonies on rivers have caused levels of toxic methylmercury to skyrocket, even if temporarily.

In the 1800s, beavers played a large role in what would eventually become the global economy. Fur traders traveled far and wide with their prized beaver hides, and as a result, populations dwindled to dangerously low levels. At one point, during the peak of beaver fur demand, the population of Eurasian beavers on the continent of Europe was no more than 1,200 individuals.

Now, in Sweden alone, there are more than 130,000 beavers, and a network of new dams to go with them. The beaver dams are quite a sight to behold, but they have been found to negatively affect the chemistry of the water in the rivers.

Methylmercury, converted from regular mercury, forms as beaver dams alter the sediments, water flow, dissolved oxygen concentration and temperature of the water.

Over a two-year study period, researchers sampled water downstream from 12 new beaver dams and found that methylmercury levels were 3.5 times higher than the water upstream.