A chemical applied to Grand Lake St. Marys reduced the amount of phosphorus in the western Ohio lake, state officials said yesterday.
Phosphorus feeds the toxic blue-green algae that have plagued the lake — and others across the state — for years.
A report the state released yesterday shows that aluminum sulfate, or alum, that was sprayed in June over the central 4,000-acre region of the 13,000-acre lake reduced phosphorus in that zone by as much as 56 percent.
“The target was to get to a 50 percent reduction,” said Scott Nally, director of the Ohio Environmental Protection Agency. “We got a 56 percent reduction, which was absolutely exciting.”
The effect on the algae itself isn’t as clear. Data in the 37-page report, prepared by Seattle-based consultant Tetra Tech, showed that chlorophyll-a, one measure of algae in water, didn’t drop with the phosphorus levels.
Chris Abbruzzese, an Ohio EPA spokesman, said concentrations of the liver toxin microcystin, which is produced by the algae, didn’t reach levels as high as those recorded last summer.
In 2010, the state warned visitors not to touch the lake water, fish or take boats out.
Blue-green algae, also called cyanobacteria, are common in most Ohio lakes and streams, but officials say phosphorus in manure that rains washed off nearby farm fields helped toxic algae grow and spread across Grand Lake each summer since 2009.
This was the first large application of alum at Grand Lake. A test run in the fall of 2010 found that alum reduced phosphorus levels at two test sites by 50 to 60 percent and produced no change at a third.
Officials also said yesterday that 210,000 cubic yards of phosphorus-laden sediment were dredged from the lake bottom this year. Also removed were more than 13 tons of rough fish, including carp, gizzard and shad, which can stir up phosphorus from the lake bottom.