Contact: Dane Konop Mike Quigley (GLERL)
Tiny shrimp-like animals called amphipods that are normally found in bottom muds of healthy lakes were absent in samples taken in November at a monitoring site on southern Lake Michigan, according to NOAA's Great Lakes Environmental Research Laboratory in Ann Arbor, Mich.
Routine monitoring of the abundance of these environmentally sensitive organisms at forty sites in Lake Michigan's southern basin provides researchers with a reliable measure of the lake's health.
While NOAA scientists have not yet determined the exact cause of the disappearance of amphipods at the site five miles off St. Joseph, Mich., they suspect it is linked to the introduction of zebra mussels in southern Lake Michigan in 1989, severely limiting food available to the amphipods.
Since amphipods normally make up to 70 percent of the living biomass
in a given area of healthy lake
"What's happening is energy that used to support amphipod growth is now being turned into zebra mussel tissue," says Nalepa. "Many species of fish, and particularly young fish, readily eat amphipods, but few species can use zebra mussels for food. There's concern that such a short circuit in the food chain could lead to declines in a number of fish, including perch, alewives, sculpin, bloater and smelt, with possible secondary effects on trout and salmon predators."
Data collected in the early 1990's indicated that the declines have been concentrated over a 5-mile-wide strip of lake bottom extending along the eastern Lake Michigan shore from near Chicago at the southern end to St. Joseph.
"Although amphipod populations declined by 60 to 90 percent in the early 1990's, there were still at least some of these animals left. When we picked through samples from the St. Joseph site in early November, we couldn't find a single amphipod. We just couldn't believe it," Nalepa said.
"During the 1980's, that site had 9,600 amphipods living on every square meter of lake bottom," Nalepa said. "Now, they're all gone. We're now wondering about how extensive this dead area might be. We hope that additional sampling planned for 1998 can provide the answers."
To sample the lake bottom, Nalepa uses a device called a "Ponar grab," a steel shovel-like device that is lowered by cable to the lake bottom from the lab's research vessel Shenehon to retrieve a measured scoop of mud. Once aboard the ship, the sample is then washed through a fine sieve to strain out any animals living in the mud.
While other organisms are still present in the mud, they are not as
readily fed upon by fish as are
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NOTE TO EDITORS: The Great Lakes Environmental Research Laboratory,
located at 2205
A map of amphipod abundance in southern Lake Michigan during the 1980's
- 90's can be found at: