Wired has a nice piece explaining the phosphorus issues in Lake Erie:
Maumee is the largest watershed in the Great Lakes system. It runs right through the Breadbasket of the Midwest, an intensively farmed area. Satellite views show thousands upon thousands of little boxes of green; the highly productive farms and fields of Ohio, Michigan, and Indiana.The Maumee watershed has to be one of the most intensively tiled and ditched watersheds in the country. Most of those ditches have little or no riparian area to provide a buffer from field runoff. Then, there's this:
The Maumee watershed is such productive farmland because it was once a swamp; the Great Black Swamp. Drainage tiles allowed removal of water from the surface of the soil, and made wonderfully rich swampland available for farming. Drainage tiles also collect up runoff and deliver it very efficiently to streams and rivers....
The problem is not as simple as “bad farmers”; farming best practices actually use fertilizer highly efficiently. Only about 2 percent of applied fertilizer is lost. The problem is volume; that many farms, golf courses, and suburban lawn owners applying fertilizer adds up, even if only a small amount washes off each allotment.
Chemistry plays a role as well; changes in the type of fertilizers sold make them more soluble (“bio-available“), and easier for plants (including algae) to take up and use. So while the total tonnage of phosphorous entering the lake is much smaller, the problem of too much phosphorous persists.
High amounts of rainfall create bursts of runoff and a sudden pulse of fertilizer. And extreme precipitation events (heaviest 1 percent of all rain events) are increasing; precipitation in downpours has increased by 37 percent in the Midwest since 1958.The Maumee watershed has some of the heaviest, most poorly drained soils in the United States, and thus will have greater amounts of runoff when high amounts of rainfall occur. Increased precipitation will result in even greater increases in runoff, as the soil can't hold any more water than it did when precipitation events were less intense.
Finally, the depth of the Western Basin plays a big part in the algae problem:
Lake Erie is different from the other Great Lakes because it’s so shallow. The western end of the lake (where Toledo is) has an average depth of only 24 feet. The water warms quickly, and it’s a great place to live if you’re a blue-green algae (cyanobacteria). Say, toxin-producing Microcystis. Microcystis has the ability to control its buoyancy; it can sink or rise to the top of the water at will to chase the sunlight.These are some of the exact same issues that caused massive problems at Grand Lake St. Mary's, except the problems there were magnified by the massive population of livestock, and the immense amounts of manure applied to the land in the watershed. Farmers have to become very proactive on this issue if they want to avoid the heavy hand of regulation. In other words, we have a short time to get our shit in gear, or the government will be forced to bitch slap us.