The green, bottom-dwelling alga called Cladophora glomerata first choked lake waters in the mid-20th century when humans discharged large amounts of phosphorus from agricultural runoff into the lakes.
Then the Great Lakes Water Quality Agreement brought tough new regulations that limited phosphorus and Cladophora all but disappeared.
Now it's back, researchers at Michigan Technological University report, thanks this time to billions of exotic zebra mussels that have created a perfect habitat.
The mussels, as filter feeders, have clarified the Great Lakes water, allowing more sunlight for Cladophora to grow in areas that were once too dark, the researchers said.
The mussels also excrete a type of phosphorus that Cladophora love to feed on, and the mussels' hard shells covering the sandy lake bottoms provide solid real estate where the algae can grow.
Michigan Tech's Robert Shuchman and his research team are helping resource managers survey the extent of the Cladophora problem.
Using remote-sensing data from satellites the can measure "radiance," or reflective brightness, to distinguish Cladophora beds from areas where the lake bottom is clear.
"By doing this, we can map Cladophora in a straightforward way," he Shuchman said.
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