Divers found tens of millions of dead mollusks at Cassville, Wis., and Cordova, Ill., in August and September. The Army Corps of Engineers found a 30-foot-by-50-foot pile of zebra mussel shells at depths of 8 feet near the lock at Dresbach, Minn.
At the same time, native Higgins' eye pearly mussels appear unaffected and healthy.
The zebra mussel, a fingernail-sized pest that was believed introduced into inland waters through the ballast of ocean-going vessels, has been plaguing the Great Lakes water system for more than a decade. Zebra mussels kill off native species, clog water intakes and spread by attaching themselves to the bottoms of vessels.
John Sullivan, a Wisconsin Department of Natural Resources water quality expert, speculated high water temperatures this summer coupled with prolonged flood conditions in the spring put undue stress on the population, leading to the massive die off.
Jim Stoeckel, a biologist with the Illinois Natural History Survey, said the zebra mussel population in the Illinois River also appears to be suffering, possibly from high sediment loads.
In Michigan, however, the zebra mussel appears to have virtually wiped out the diporeia, a half-inch-long, shrimp-like crustacean that has been in the Great Lakes for thousands of years and serves as a food source for whitefish, smelt, alewives and chubs.
The zebra mussel also appears to be attaching to each other to form vast reefs along the sandy bottoms of Lake Michigan and Lake Huron, as deep as 100 feet, filtering out light for the tiny plants on which the mud-dwelling diporeia feed, according to Thomas Nalepa, a biologist with the Great Lakes Environmental Research Laboratory in Ann Arbor, Mich.
"The exact link is unclear but wherever zebra mussels are present, diporeia are not," he told a scientific conference this week in Traverse City, Mich.
Nalepa said the decline in diporeia at more than 200 sites he monitors was in the 90 percent to 100 percent range. The species began declining in 1992 near Lake Michigan's southern tip near Chicago.
Zebra mussels are native to the Black Sea. They first appeared in the Great Lakes in the 1980s. Scientists say the creature's behavior has changed since the introduction.
"You're used to seeing zebra mussels attach to things but now they just attach to themselves, forming reefs right on the sand," he said. He estimated the reefs eventually could cover 75 percent of the lakes' bottoms.
In sections of the Mississippi River, zebra mussels number as many as 20,000 per square yard. They crowd out native mussels by not only competing for the same food sources, but by attaching to the shells of native species, preventing reproduction.