A recent story published in The Atlantic discusses blue-green algae and Grand Lake St. Marys.  Jessica Marshall writes:  

"Every summer, as temperatures rise, "blooms" of cyanobacteria like the one in Grand Lake St. Marys 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. The blooms occur in nearly every state, peaking in August and September. 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."