Thick, slimy algae blooms covered Lake Erie once again last week, stretching for more than 700 square miles.
Such bright-colored blooms have increased in size and frequency since the 2000s, according to the New York Times, mostly because of heavy fertilizer use on nearby agricultural land. Rain causes the fertilizer to run off the land into rivers that empty into Lake Erie.
The lake now experiences algae blooms every summer, according to the Cleveland Plain Dealer. Similar blooms are happening in lakes around the world as a result of warming temperatures and increased fertilizer use—a result of greater agricultural needs to feed a growing global population.
Part of the blooms—cyanobacteria—can produce a toxin that seeps into drinking water and poisons local ecosystems. The toxins, called microcystins, can cause liver damage among other adverse health effects in humans.
However, not all algae blooms rise to the level of toxicity. The lake’s current algae blooms are being monitored and their toxicity as thus far remained low.
Still, large algae blooms can still hurt the lake’s recreational economy, centered around activities like fishing and going to the beach, because people are still turned off by the sight and smells of decaying algae.
David Spangler, the vice president of the Lake Erie Charter Boat Association, described the blooms to the New York Times as a musty-smelling, skin-like substance on the water’s surface, so thick that people could write their names in it.