This story is part of special National Geographic News series on global water issues.



When antidepressant pills get flushed down the drain, they do more than create happier sewers.



Scientists in Erie, Pennsylvania, have found that minute concentrations of fluoxetine, the active ingredient in Prozac, are killing off microbial populations in the Great Lakes.



Traces of antidepressants such as Prozac have been found in both drinking and recreational water supplies throughout the world, in quantities experts say are too dilute to affect humans but which have been found to damage the reproductive systems of mollusks and may even affect the brains of animals like fish.



(Read more in National Geographic News: “Prozac Pollution Making Shrimp Reckless.”)



Killing off bacteria might seem like a good thing. "Your immediate thought is, 'well, that's good, because they're not supposed to be there anyways," said Mercyhurst College microbiologist Steve Mauro, whose team found fluoxetine in low doses in water near Lake Erie's beaches. "But what about all the other bacteria that are supposed to be there and part of that ecosystem?"



Treating clean lake water with similar strength doses killed off E. coli and enterococcus bacteria, both of which can cause serious infections in humans.



The fluoxetine found in Lake Erie is at very low levels—about one nanogram per liter of water, Mauro said. "It doesn't appear to be at a level that would be harmful to humans," or invertebrates, for that matter, though Mauro suspects that fluoxetine combined with other chemicals could be having a cumulative effect on the lake's ecosystem.