Those keeping an eye on the water quality of Ohio’s lakes say they expect toxic blue-green algae will make a comeback this summer.

The Columbus Dispatch reports that warning signs were posted last year at 20 public lakes, pond and beaches. David Culver, who works at an Ohio State lab that studies freshwater ecology, predicts this year will be the same because heavy rainfall this spring has sent nutrients that supply algae into waterways.

The algae can grow toxic from manure, fertilizer and sewage runoff. Emeritus Director David Baker of the National Center for Water Quality Research at Heidelberg University says total runoff this year already exceeds the amount from last year.

Warning signs went up May 19 at Ohio’s largest inland lake, Grand Lake St. Marys in western Ohio.