Blooms occur in Lake Erie’s western basin, Lake Michigan’s Green Bay and Lake Huron’s Saginaw Bay.

North America’s Great Lakes are the largest source of fresh surface water in the world. They are essential for cities, industries, and agriculture in eight states in the United States and one province in Canada. Large blooms of algae plagued the lakes in the 1960s, but largely disappeared by the 1980s due to efforts to clean up phosphorus pollution. By the late 1990s, however, the blooms reappeared and are now an unsightly and dangerous problem.

The blooms close beaches and harm fisheries. Some, like the blooms of cyanobacteria in Lake Erie and Green Bay, are toxic, posing a threat to drinking water supplies. Others, like the blooms of bottom-dwelling algae along Lake Michigan’s Sleeping Bear Dunes National Lakeshore, wash up in fetid mats on beaches and can harbor diseases that infect waterfowl. While the size of the blooms vary from year to year due to factors like currents, water temperature, and the amount of phosphorus in the water, they tend to stay in the same general locations—many times spreading outward from the mouth of a major tributary to the lake, such as the Maumee River in Ohio.

This map was created by Erin Aigner for Circle of Blue. Click image to enlarge.

This map was created by Erin Aigner for Circle of Blue. Contributors include Codi Yeager-Kozacek and Aubrey Ann Parker of Circle of Blue. Reach Circle of Blue’s data team at info@99.198.125.162/~circl731.

Choke Point: Index is produced in collaboration with Google Research, Qlikview, and the Columbia Water Center, with financial support from the Rockefeller Foundation.