A virulent algae bloom feeding on agricultural nutrients has transformed the water, yet again, into something green and oozing.

Visitors to Lake Erie should be forgiven if they believe they’ve stumbled upon a vast, fetid swamp. A huge portion of the lake has turned gooey green, thanks to a thick carpet of algae that’s been expanding since mid-July.

Here’s a satellite shot of the infestation from last week with a boat cutting through the ooze:

(USGS/NASA/Joshua Stevens)

Zoom out and you’ll find the whole western part of the lake looking like a victim of a Nickolodeon Gak spill:

(USGS/NASA/Joshua Stevens)

A photography service helping NOAA and others research these types of algal blooms captured nauseating views of the sludge in a confrontation with the coastline:

(Zachary Haslick/Aerial Associates Photography via NOAA)

Algae blooms occur regularly in Lake Erie in the late summer and fall—government meteorologists even forecast and track them—though this year the stuff’s breadth is particularly impressive. So is its potential wallop to health. The algae, called cyanobacteria, contains toxic microcystins that can cause skin blistering, liver damage, and flu-like symptoms in humans. It has proved fatal to livestock. There’s also research, for whatever it’s worth, that certain chemicals in cyanobacteria make fish spontaneously change sexes.

So what’s to blame for the gunk? Well, consider the conditions that cyanobacteria need to thrive—warmish water and nutrients like phosphorous and nitrogen. The farming industry around the Great Lakes uses great amounts of these chemicals, a lot of which get washed downstream into larger bodies of water. Back in the 1960s, monstrous blooms were common due to large nutrient releases from agricultural and industrial facilities as well as sewer systems.

The Great Lakes Water Quality Agreement between Canada and the U.S. reigned in most of the pollution in the ‘70s, but in recent years the algae levels have crept back up, partly due to farming practices that have continued to allow fertilizer seepage. The resulting blooms kill marine life, cost more than $80 million annually in lost fishing and tourism activity, and put drinking-water managers on edge. One prominent bloom in 2014 had Toledo telling half-a-million people not to ingest tap water, because boiling doesn’t destroy microcystins. So far this year, the concentration of algae hasn’t grown so bad to require such advisories in local communities.

There’s evidence that climate change is aiding the resurgence of troublesome algae here and around the world. The warming atmosphere holds more moisture and delivers more-intense rain events, which in turn push more nutrients into lakes and coastal waters. You can see how Lake Erie’s green streak evolves after storms in these satellite images from a 2011 bloom, showing sediments flowing from the flooded Maumee River (at middle-left) into Lake Erie and fueling the spread of toxic algae.

(NASA/University of Wisconsin-Madison)

Algae love warm water, and with water temperatures rising across the globe, these baleful blooms likely will become more common. It’s something Great Lakes environmental officials will have to think hard about: Some climate models have predicted that winds in the region will decrease in the future, and without them, algae find a perfect environment to just chillax and breed. Indeed, as to the ongoing bloom NASA writes: “Forecasts for the near-term suggest that winds will be too calm for surface waters to mix much. The lack of mixing allows for the formation of scums—areas where cyanobacteria clumps together into floating mats.”

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