I grew up in Saint Marys, but I've never heard of Beaver Creek. No, I'm not talking about the city of Beavercreek, a part of the growing body that is the Dayton, Ohio metro area. I live about a mile and a half away from there.
I mean the creek upon which Celina, Ohio's water treatment plant is situated. The one they've been putting phosphorus in.
Going back to the original post on the lake; "In the last year before the planktothrix poisoning, the Natural Resources Conservation Service found high phosphorous concentrations in Chickasaw Creek." Chickasaw Creek is a tributary to Lake St. Marys.
The Celina story does not call Beaver Creek a tributary, but it does not necessarily have to be one for this to be a problem. Google searches, and looking at Google's map, do not bring up the name of the creek much. It is not a big body of running water. If it rains a lot, and there is flooding (frequent occurrence in Celina,) it will be overwhelmed by the flow of water, and all of the excess water in that area will eventually end up in the lake.
This makes dumping phosphorus into the creek a self-defeating act for the water wreatment plant, as Celina gets its water from Lake Saint Marys. In other words, first they're taking it out, and then in an only slightly roundabout way, they're putting it back. In this way, the issue is perpetuated, rather than solved.
For more technical info: Beaver Creek and Grand Lake St. Marys Water Quality Information
The bottom line: The way things have been done in the area is going to have to go the way of the Model-T. Saint Marys and Celina are going to have to modernize their way of thinking. As long as local communities continue shape up their act at the behest of the EPA, the lake will survive. If not, the largest hand-dug body of water in the U.S. will become nothing but a giant puddle of stagnant water.
Satellite image from 2000 (i frame not embedding properly)