ODNR News Release

Visitors to Grand Lake St Marys will be able to enjoy the water this weekend without a water quality advisory for the first time in more than 10 years, according to the Ohio Department of Natural Resources (ODNR).

“It’s really encouraging to see the clear waters at Grand Lake St Marys right now,” said ODNR Director Mary Mertz. “We’ve been very focused on improving water quality at this beautiful lake and we are happy to see this bright start to the summer season.” 

Grand Lake St Marys, located in Auglaize and Mercer counties, has been plagued by algae for more than a decade due to nutrient-rich runoff into the lake and phosphorus-laden sediment in the lakebed. Phosphorus and other nutrients contribute to the growth of harmful algal blooms. 

For years, recreational health advisories, known as red-flag advisories, have been in place at the lake warning visitors to avoid contact with the water. Those advisories are issued based on the elevated levels of microcystin in the water. Microcystin is one of the most common toxins produced by harmful algae and can make humans and animals sick.  

The state considers recreation in water with an algae bloom to be safe for all ages only if microcystin levels are below 8 micrograms per liter. Red-flag advisories are issued when microcystin levels are at or above 8 micrograms per liter. Current levels at the lake are less than 0.4 micrograms per liter at all four state park beaches, so there are currently no health advisories in place at the lake. 

Wetland treatment trains and agricultural best management practices in the region are playing a role in the water quality improvements we are seeing this spring.  These practices remove nutrients from water before it runs into the lake.  

It is likely that we will see advisories return to Grand Lake St Marys later in the recreational season. With increased precipitation and rising lake temperatures, the risk for toxic algal blooms increases.  

The Ohio Department of Natural Resources will continue to monitor water sampling results provided by the City of Celina and visually inspect our beaches for evidence of algal blooms. If a bloom is observed, we will collect a sample and test the water. If toxin levels rise, the public will be notified online through the BeachGuard website and by signs at the lake.