In a new report out today, the Food & Environment Reporting Network (FERNnews) highlights how toxic blue-green algae–or cyanobacteria–is thriving in freshwater across the U.S., creating health risks. The story appears online at the Center for Investigative Reporting and The Atlantic.

“Every summer as temperatures rise, ‘blooms’ of cyanobacteria develop in lakes and rivers across the country, turning waters intense green, and coating swaths of their surfaces with putrid-smelling blue-green algae that looks like pea soup,” writes FERNnews reporter Jessica Marshall. “The blooms occur in nearly every state, peaking in August and September, and—though no national agency tracks the blooms—experts say they are getting worse, driven by fertilizer and manure run-off into lakes and streams combined with a warming climate.”

Marshall explains that cyanobacteria occur naturally in lakes, typically at low concentrations that are not harmful and not visible. But when levels of key nutrients—particularly phosphorus—in a water body soar and combine with hot temperatures and stagnant water, the organisms thrive. Under these conditions, they outpace growth of other types of algae and streak or coat the water with bright, sometimes iridescent, blooms of green or blue-green cells.

Many people near the blooms complain of the smell produced by ammonia and hydrogen sulfide as the cyanobacteria rot. “But worse,” Marshall reports, “Under conditions that scientists don’t completely understand, cyanobacteria can produce toxins that cause asthma-like symptoms, severe vomiting or diarrhea, or irritated skin or eyes.”

Marshall interviews Dan Jenkins, who was partially paralyzed after washing bright, stinking algae blooms—or green slime—off his dog, Casey, after he had swam in a tainted lake in Ohio. The dog later died from its exposure–one of at least 10 algae-related dogs deaths reported nationwide in the last two years. In Wisconsin alone, 98 people have reported illness from blue-green algae exposure over the last three years–though experts agree that many cases go unreported or misidentified.

In looking for the source of the problem, Marshall reports, experts point to agriculture: Phosphorus-laden fertilizer and manure can wash directly into waterways, and eroding sediment from farmlands carries the substance too. In addition, flows from sewage treatment plants and urban storm drains, runoff from lakeside lawns, and discharges from industries such as pulp and paper mills can also contribute phosphorus to streams and lakes.