On Aug. 2, the City of Toledo, Ohio issued a water use ban for roughly 500,000 residents after chemists detected toxic levels of microcystin in the public water supply. Microcystin is a toxin produced by harmful algal blooms caused by the overuse of nitrogen and phosphorous fertilizers. Large amounts of excess fertilizers run off into waterways during rainstorms. Exposure to microcystin can cause diarrhea, nausea, liver dysfunction, and nervous system damage. Beyond the public health risks, harmful algal blooms also negatively impact ecosystems and burden the economy.