It was never a serious nuisance, however, until the mid-twentieth century, when humans began discharging phosphorus into the Great Lakes in a big way. That led to an unprecedented number of huge, gooey mats of Cladophora (pronounced klah-DAH-for-uh) covering 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, thanks to tough new regulations that limited phosphorus. Now Cladophora is back with a vengeance, thanks this time to billions of exotic zebra mussels that have created its perfect habitat.
First, the filter-feeders clarify the Great Lakes water, allowing in more sunlight--and allowing Cladophora to grow in areas that were once too dark. Second, they excrete a type of phosphorus that Cladophora love to ingest. And third, their hard shells covering the sandy lake bottom provide solid real estate where the algae can attach.
Is there hope? Maybe. With funding from the Environmental Protection Agency’s Great Lakes Restoration Initiative, Robert Shuchman, codirector of the Michigan Tech Research Institute, 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. With thousands of miles of Great Lakes shoreline, no one had a good estimate of the extent of the Cladophora beds.
Shuchman’s team uses 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, 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. The researchers 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.
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.
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.”
Robert Shuchman | Newswise Science News
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