- Associated Press - Monday, August 4, 2014

Hundreds of thousands of people in Toledo, Ohio, and nearby southeastern Michigan were unable to use tap water from Saturday until Monday morning because of unsafe levels of a contaminant called microcystin in Lake Erie. Here are questions and answers about the situation:

Q. What is microcystin?

A. A toxin produced by microcystis, a type of blue-green algae, or cyanobacteria. Only certain species of blue-green algae form toxins, for reasons that aren’t fully understood.

Q. How dangerous is it?

A. Microcystin can produce hives or blisters from direct contact with the skin. Swallowing it can cause headaches, fever, nausea, vomiting and diarrhea. Large-scale ingestion can damage the liver.

Q. Why is it such a big problem in Lake Erie?

A. Algae can be found in virtually any lake. But Erie is the shallowest and warmest of the Great Lakes - its average depth is 62 feet - making it the most biologically productive. It is loaded with nitrogen and phosphorus, the primary nutrients of algae. Phosphorus levels are especially important in determining how much algae form in a water body.

Q. Are farms the primary source of the phosphorus that causes blue-green algae?

That question leads to lots of finger-pointing. A February report by the International Joint Commission, a U.S.-Canadian organization that advises both nations on the Great Lakes, said municipal sewage plants were the primary source of Lake Erie phosphorus prior to a 1972 agreement to improve water quality. Since then, the report said, the leading culprit is runoff from farm fields, lawns, city streets and parking lots. Even deposition from the atmosphere makes a small contribution.

Farm groups contend municipal sewage remains a big factor - particularly the Detroit treatment plant, which discharges into two Lake Erie tributary rivers. Studies show the Detroit River does contribute a large share of the lake’s phosphorus, although concentrations are significantly higher in the Maumee River, which drains agricultural areas of northwestern Ohio and flows into Lake Erie at Toledo.

Q. Does Lake Erie have a history of algae pollution?

A. Some experts described Erie as virtually sterile in the 1960s, partly because of “dead zones” caused by decomposition of algae that sucked oxygen from the water, leading to repeated fish kills. Improvements in sewage treatment and limits on phosphorus in detergents boosted water quality. But the blue-green algae that generate microcystin turned up in the mid-1990s, and the problem has worsened. In summer 2011, the lake’s largest bloom on record coated nearly 20 percent of Erie’s surface.

Q. What caused recent outbreaks, and why are they more severe than before?

A. Scientists believe three factors make the present situation different.

First, a greater portion of the phosphorus washing into the lake is “dissolved reactive phosphorus” - the form that stimulates algae growth the most. That may be explained by changes in agricultural practices. Farmers have made greater use of “no-till” cultivation and application of manure and chemical fertilizers directly onto the land’s surface, instead of working them into the soil as before. That makes it easier for the phosphorus they contain to flow directly into creeks that feed the lake and dissolve there instead of interacting with the soil.

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