Blame agricultural run-off.
Algal Blooms May Become the Norm in Lake Erie: Scientific American
The Native Indians of Ohio called the area that is now northwestern Ohio and southeastern Michigan the "Great Black Swamp" -- a land of marshy wetlands. The early pioneers were able to dredge the land and turn it into the rich, productive region it is today. Corn and soybeans dominate in the region, and high prices for both commodities in recent years have encouraged farmers to keep sowing more rows, said Joe Logan, director of agricultural programs for the Ohio Environmental Council.
"There is a compelling economic interest for farmers to maximize productivity," Logan said.
Below the surface of the fields lie networks of perforated tubing, which pull excess water from the soil and send it to the lake, rich in phosphorus, Logan said.
Before the 1970s, phosphorus runoff to Lake Erie's western basin was mostly due to municipal sewers and industry, said David Baker, who founded the National Center for Water Quality Research at Heidelberg University in Tiffin, Ohio, in 1969. By 1980, the phosphorus from agricultural runoff was almost twice the amount from municipal sources, he said.
The patterns also changed. While municipal sources of pollution tend to be consistent throughout the year, agricultural runoff increases tremendously in the spring with the swelling of rivers and streams from the rain.