Cladophora glomerata
The oozy, green, bottom-dwelling alga called Cladophora glomerata has squished around toes about as long as people have been wading in the Great Lakes. Photo by Krystal Johns
Robert Shuchman
Robert Shuchman uses satellite data to pinpoint the location of Cladophora in the Great Lakes. Photo by Michael Blanchard
Martin Auer
Martin Auer was among the first to establish a link between phosphorus and an overgrowth of the nuisance alga.
“Quagga mussels have gobbled up the food supply for Asian carp in mid-Lake Michigan, ‘but there’s plenty of Cladophora near shore for them to eat.’”

Return of the Slime

by Marcia Goodrich

The oozy, green, bottom-dwelling alga called Cladophora glomerata has squished around toes about as long as people have been wading in the Great Lakes. It was never a serious nuisance, however, until the mid-twentieth century. That’s when an unprecedented number of huge, gooey mats of Cladophora (pronounced klah-DAH-for-uh) blooms uprooted and drifted ashore, fouling entire beaches with a thick layer of rotting muck.

Then came the Great Lakes Water Quality Agreement, and the mats of Cladophora all but disappeared. Says Robert Shuchman, codirector of the Michigan Tech Research Institute, “It was a great success story.” It was also a short one. After thirty years in the wings, Cladophora is back on center stage.


The story begins in the 1970s, when Martin Auer was a PhD candidate at the University of Michigan. His advisor asked him to check out the relationship between the phosphorus-rich effluent flowing from a sewage treatment plant and the jungle-like growth of Cladophora thriving nearby. Auer, now a professor of civil and environmental engineering at Michigan Tech, was able to demonstrate that lowering phosphorus levels from the effluent got rid of nearly all the Cladophora. Through this work, Auer became a respected expert on one of the biggest problems afflicting the Great Lakes.

Guided by Auer’s findings, the US and Canada developed regulations that would slash the amount of phosphorus entering the Great Lakes. Soon, the massive algal blooms faded away.

“People said, ‘We’re done,’” Auer remembers. “‘There’s nothing else to do. That’s the end of it.’” They appeared to be right. Auer calls 1985 to 2005 “the Dark Age of Cladophora.” “It seemed,” he said, “like there was no more Cladophora work to be done.”

Of course, Cladophora was still around; it just wasn’t a nuisance. “It’s not an invader,” Auer notes. “It’s always been in all of the Great Lakes except Superior.” A filamentous green alga, it attaches to anything solid, like rocks, and floats upward toward the sun. “If I were to pick you up and dunk you in the lake, your hair would hang like Cladophora,” he says.

Then, a generation after the Cladophora problem had supposedly taken its final bow, it was back with a vengeance. Vast stretches of lake bottom in Lakes Erie, Huron, Ontario, and Michigan were coated. “The stuff was washing up and clogging the cooling water intakes for nuclear power plants,” Auer says. “The power plants had to shut down.”

Initially, no one understood what was going on. “We’d been removing phosphorus, and everything was supposed to be getting better,” Auer says. As it turned out, the reason everything was now getting worse came in the form of another pesky species.

Practically no one in the Great Lakes region had even heard of zebra mussels during the first Cladophora explosion. But in the intervening years, the oceanic invaders hitched rides in the ballast tanks of sea-going vessels and established themselves in all of the Great Lakes, save Superior.

They not only clogged water intake pipes and disrupted the food chain, “they changed the Great Lakes in three ways that benefit Cladophora,” says Auer. “It’s not your grandmother’s ecosystem anymore.”

First, zebra mussels filter particles out of the lakes, clarifying the water and letting sunlight penetrate deeper. It’s like turning on a grow light. “Now, where Cladophora used to grow at depths of ten feet, they now grow at forty-five,” said Auer. “You have Cladophora growing where it never could before.”

Second, the mussels are whipping up Cladophora’s favorite food. “Rivers carry a type of phosphorus that Cladophora can’t use, which used to go out to the center of the Great Lakes,” Auer said. “Now come the zebra mussels, and they filter out that phosphorus and spit it back in a form that’s perfect for Cladophora. It’s like living upstairs of the bakery.”

Finally, the zebras are creating new real estate. Billions of zebra mussel shells coat the once-sandy lake bottom, providing a hard surface where Cladophora filaments can attach.

“So you have a triple whammy,” says Shuchman. And thus far, no one has found a way to evict zebra mussels from the Great Lakes. Controlling twenty-first century Cladophora is turning out to be far more complicated than keeping excess phosphorus out of the ecosystem.

In 2009, the US Environmental Protection Agency took aim at the myriad problems plaguing the Great Lakes through the Great Lakes Restoration Initiative. With funding from the initiative, Shuchman and his research team are helping resource managers get their arms around the Cladophora problem.

“The EPA asked us to find out where Cladophora is concentrated,” Shuchman said. It seems like an easy enough question if your boat’s propeller is encapsulated in slime, but with thousands of miles of Great Lakes shoreline, no one had a good estimate of the extent of the Cladophora beds.

To find out, Shuchman and his team are using remote-sensing data from satellites. They measure “radiance,” or reflective brightness, to distinguish Cladophora beds from areas where the lake bottom is clear. That’s relatively easy at a constant depth, but radiance drops as water gets deeper.

To account for the difference, Shuchman integrated the satellite data with information on the lake bottoms and developed an elegant algorithm that compensates for the depth of the water.

“By doing this, we can map Cladophora in a straightforward way,” he said. To verify their results, they use ground truth, i.e., up-close sensing: researchers boat along the shoreline and visually check for Cladophora, often using a remote-controlled mini-submarine camera. These surveys showed their remote-sensing analysis to be about 90 percent accurate.

Shuchman’s team will also track the historical ebb and flow of Cladophora by applying the algorithm to satellite images that go back decades.

Armed with this information, resource managers will be able to locate Cladophora beds and tell if their cleanup efforts are working. The EPA is already using their data to track the health of the Great Lakes. Auer and Shuchman are also considering ways to track the algal mats, which typically slough off the lake bottom in midsummer, and possibly block them before they land on beaches or in the cooling water intakes of nuclear power plants.

The team members will map all US waters in the Great Lakes that are optically visible—those parts where light can be reflected off the bottom. Already they have determined that sunlight is reaching much more of the lake bottom than in years past, thanks to the mussels’ insatiable appetite for water-clouding plankton. “I hate zebra mussels, but they have made it possible for us to look into deeper water,” Shuchman said.

The team has already finished mapping Lake Michigan. They have found Cladophora on 591 square miles, or about a third of the optically visible area. Ironically, Cladophora is superabundant near Sleeping Bear Dunes National Lakeshore, where twenty-inch-thick algal mats lie just a few yards off shore. “That place is full of Cladophora,” Auer said. “But Good Morning America named Sleeping Bear Dunes the most beautiful place in America.”

It wouldn’t be so beautiful if those mats washed up on the miles of white sand beaches, he added. In addition to creating a repulsive viewing experience, rotting Cladophora provides ripe conditions for avian botulism and has been implicated in the poisoning deaths of thousands of shorebirds.

The abundance of Cladophora could have even broader implications. “It’s a little bit scary,” Shuchman said, in part because the Michigan Department of Natural Resources reports that the exotic Asian carp can eat Cladophora. The voracious fish have infested the Mississippi River system, and many fear they will expand their range into the Great Lakes via the Chicago Shipping Canal.

The filter-feeding quagga mussels have essentially wiped out their potential food supply in the middle of Lake Michigan, “so the carp will never survive in deep water,” said Shuchman. “But there’s plenty of Cladophora near the shore for them to eat.”