CELINA, Ohio (AP) --Signs were going up on the beaches of Ohio's largest inland lake Thursday warning visitors that toxic algae blooms are back.
Toxic blue-green algae is again making swimming hazardous at Grand Lake St. Marys in western Ohio. It's been a recurring problem at the 20-square-mile lake between Dayton and Toledo.
State officials have been testing near lake beaches this week after measurements of a liver toxin associated with the algae began to increase last month near where the city of Celina draws water into its treatment plant, according to The Columbus Dispatch. By the end of April, readings of microcystin measured four times higher than the state's safety threshold.
Ohio Department of Natural Resources spokesman Matt Eiselstein said exact measurements are still being calculated, but the readings are "definitely over the safety level."
Blue-green algae are common in most Ohio lakes, fed by phosphorus from manure, fertilizers and sewage that rain washes from farm fields into nearby streams. As many as 19 public lakes, including Erie, have been tainted in recent years by toxic algae.
Algae grew so thick in Grand Lake in 2010 that the state warned people not to touch the water. Officials say it likely caused seven people to get sick that year.
The city of Celina spends about $450,000 a year to control algae at Grand Lake, and the state has spent more than $10 million trying to treat it. The Ohio Environmental Protection Agency says it could take years to reverse the situation.
The phosphorus runoff from area farms is such a summertime problem at Grand Lake St. Marys that nearby farmers now face state-mandated limits on the manure they spread on fields.