From Scientific American
The rains come and water the spring shoots of another bounteous Midwestern corn crop in Ohio, Indiana and Michigan. The rains also wash phosphorus off farm fields and into creeks, streams and rivers. The waters flow into the shallowest of the Great Lakes—Lake Erie, which is just 18 meters deep on average and far shallower on its western edge. All that phosphorus doesn’t just help crops grow. When it reaches the lake it fuels the growth of mats of bright green algae, turning the water the color of pea soup.
Such Microcystis cyanobacteria bear poisons, at least 80 different varieties of a toxin dubbed microcystin. And when the shallow waters deliver an algal bloom down to the right water intake pipes, an entire city like Toledo is left without water.
The microcystin toxin is quite dangerous—more potent at smaller doses than powerful pesticide DDT, neurotoxic mercury and even the poison cyanide. It can cause liver damage in humans as well as vomiting, diarrhea and abdominal pain.