An algal bloom is an above normal growth of algae or related bacteria in a body of water. A harmful algal bloom, or HAB, is an algal bloom that contains a toxin that can be dangerous to touch or consume.
In Lake Erie and other freshwater areas, a HABs are caused by above normal growth of blue-green algae, otherwise known as a cyanobacteria (pronounced ‘sigh-ann-oh-back-tee-ree-ah’). The “harmful” parts are toxins; the cyanobacteria usually responsible for these explosions of growth in Lake Erie produce a chemical, microcystin, that is harmful if eaten or touched. HABs also occur in marine waters, but are caused by something else (these marine HABs are also called "red tides").
Cyanobacteria, including the few types that typically form HABs, are common. What makes a HAB is the amount of growth combined with the toxicity. Given that these HABs can be sucked up into the treatment plants that clean our water for drinking and showering, and they stop people from swimming and enjoying our waterways, researchers are trying to figure out the best way to prevent them from happening.