DULUTH (AP) -- Scientists are investigating an unprecedented algae bloom in Lake Superior and have sent samples for lab testing to determine if it's toxic.

Kayakers and other visitors to the popular lakeshore along eastern Minnesota and northwestern Wisconsin found green scummy algae last week on Lake Superior's surface.

Robert Sterner directs the Large Lakes Observatory at the University of Minnesota Duluth. He says the bloom has largely dissipated but reports suggest it stretched 50 miles along the shoreline from near Superior, Wisconsin, to the Apostle Islands.

Blue-green algae blooms have become more commonplace in Midwestern lakes and rivers. Scientists say the increase is partly because of warming water temperatures.

Researchers expect to know by the end of the week whether the bloom was toxic.