GIBRALTAR ISLAND, Ohio — Four years ago, Lake Erie algae wreaked havoc on the Toledo water system, affecting 400,000 residents in Ohio and Southeast Michigan. Known as harmful algal blooms, the deep green ooze grows by the mile each year, fed by phosphorus-rich runoff from farm fields. Though not always toxic, the eyesores gunk up beaches, choke marine life, and became far more serious in August 2014. That’s when Toledo’s water system sucked up cyanobacteria (known as blue-green algae, though it’s technically not algae) and contaminated drinking water with microcystin, a toxin that can cause liver and kidney damage. The scare prompted residents to rely on bottled water for three days and cost about $65 million, mostly in lost tax revenue and tourism.