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THE abundant fresh water of the Great Lakes helped turn America’s Midwest into an industrial powerhouse. Carmakers in Detroit, steelmakers in Cleveland, brewers in Milwaukee and makers of furniture in Grand Rapids used huge quantities of water to produce their wares. They also abused it. For almost a century they poured wastewater contaminated with metals, oils, paint and other toxins back into the lakes.

Midwesterners woke up to the damage done when the Chicago, Rouge and Detroit rivers caught fire in the 1960s, fuelled by the oily sludge in the lakes and their arteries. In 2010 the Great Lakes Restoration Initiative was born to improve water quality, clean up shorelines and restore habitats and species. These days, new factories on the lakes’ shores are viewed with suspicion. On March 7th Wisconsin’s Department of Natural Resources will stage a public hearing about the controversial plans of Foxconn, a Taiwanese maker of electronics, to draw 7m gallons of water a day out of Lake Michigan. Yet those who live near the Great Lakes are also inadvertently polluting the water.

When people take antidepressant drugs or hormonal medicines such as the contraceptive pill, or even use some grooming products, traces end up in the Great Lakes. Diana Aga, a chemist at the University at Buffalo, has found high concentrations of the active ingredients of antidepressants such as Prozac, Zoloft, Celexa and Sarafem in the brains of fish taken from the Niagara river, which connects Lake Erie to Lake Ontario.

According to national health surveys, the proportion of Americans aged 12 and over who take antidepressants rose from 7.7% to 12.7% between 1999-2002 and 2011-14. The drugs accumulate in fish. In some cases the levels of antidepressants within brain tissue are at least 20 times higher than in the water. This does not pose a danger to humans, who seldom eat fish brain, says Ms Aga. But it could well damage the ecosystem of the lakes.

Fish respond similarly to humans on antidepressants. They are less risk-averse and, it appears, happier. That seems to make them more likely to be eaten. Victoria Braithwaite, of Penn State University, worries that these sorts of changes could trigger the collapse of an entire fish population, or even seriously disrupt the biodiversity of the lakes—the largest freshwater ecosystem in the world.

A new study from McMaster University raises more concerns. It finds that bluegill sunfish, common in North America, have to burn much more energy to cope with the array of toxins that they typically encounter. They have less energy left for growth, reproduction and survival. Effluent from wastewater treatment plants does not kill the fish immediately, but its effect is insidious, says Graham Scott, one of the authors of the study.

What can be done? The molecules of antidepressants and other contaminants are too small for treatment plants to catch. Yet Ms Aga says that advanced oxidation processes can filter out many drugs and beauty products. It would be hard to update or replace more than 1,400 wastewater treatment plants around the lakes. But pressure for change could grow, especially if local industries begin to suffer. Last summer tourists visiting Niagara Falls spotted a large amount of black sludge in the river. A few months later, Andrew Cuomo, the governor of New York state, proposed to invest $20m in the wastewater system.