Algae swamped Utah Lake last week, stretching over 134 square miles. The bright green bloom is mesmerizing — and possibly toxic. And, turns out, poop is partly to blame.

This stunning (and kind of gross) aerial captured by Salt Lake Tribune photographer Rick Egan shows the algae choking the water near the Utah Lake’s marina. The bloom was discovered around July 12, and at its peak covered 90 percent the water's surface, spreading north up the Jordan River toward the Great Salt Lake. High levels of cyanobacteria caused public health officials to close the lake for further testing, and posted signs warning unsuspecting visitors to keep out.

So what made the green goo? The Utah Department of Environmental Quality blames the bloom on a possible combination of high temperatures and low lake levels that increased the concentration of phosphorus, which helps algae grow. Over 76 percent of that phosphorus arrives to the lake through discharge from eight nearby wastewater treatment plants. As UDEQ spokesperson Donna Spangler kindly puts it, “It comes from people using the bathroom.”

Public health officials closed the lake on Friday. Spangler says they’re waiting on test results to show whether or not the algae cells are actually releasing toxins in the water. There have been 500 calls to poison control, mostly from people who were out on the water and later experienced symptoms like vomiting, headache and skin rash.

Utah Lake is no outlier. According to David Caron, professor of biological sciences at the University of Southern California, freshwater blooms are becoming more frequent as temperatures warm. One solution would be for cities to upgrade their sewage treatment plants to remove nutrients before they ever hit the water. But it's expensive, so some choose to dump algae-killing toxic chemicals like copper sulphate into lakes or add clay that binds up phosphorus at the bottom, where it sits indefinitely.

The UDEQ plans to let the bloom clear up by itself. It typically takes a week or two, though the toxins can persist for days afterward—just something to think about next time you flush.