Expanding populations of zebra -- and quagga -- mussels would pose potential threats to fish-eating birds.
Birds can be put on the list of possible victims of the zebra mussels being found in Minnesota lakes.
Actually, two mussel species are likely to pose the potential problem. There are the zebra mussels you have heard about. And there are quagga mussels, a huge problem in the eastern Great Lakes but less well-known here. They are present in small numbers in the Duluth harbor and the lower reaches of the St. Louis River.
Zebra mussels have been found in Lake Minnetonka, Lake Mille Lacs and perhaps two dozen other Minnesota lakes. They will multiply, and they are very likely to infect other lakes. Quaggas, which out-compete zebra mussels, could follow.
Much of what we know about these invasive creatures comes from studies of the Great Lakes, so it's not clear just how the mussels will affect our inland lakes. The threat is significant, however, both to a lake's food chain and to birds that eat fish.
The biggest threat probably is the change the mussels could bring to lake food chains, which could impact waterbird species.
The mussels feed by siphoning water in, removing by filtration substantial amounts of phytoplankton and suspended particulate. They discharge what then is exceedingly clean and clear water.
The clear water then allows more light to enter the lakes, which can change vegetation patterns.
Lake Michigan has zebra and quagga mussels, the latter dominant.
"The mussels are causing the biggest changes we've ever seen in this lake," said Thomas Nalepa, a federal research biologist working in Michigan.
Gary Montz, an aquatic invertebrate biologist with the Minnesota Department of Natural Resources, said, "If we see a disruption in the base of the food chain by extremely high densities of zebra mussels, this could impact food for larval fish. That in turn could impact other aquatic life that depend on such fish."
That includes fish-eating birds -- loons, mergansers, grebes, cormorants, scoters and gulls.
However, Montz says, he has not seen reports that such food chain impacts on birds have happened. Most studies have been made on the Great Lakes.
Toxins and botulism
While filtering the water, the mussels accumulate toxins, pollutants and microorganisms. Those concentrations are intensified in the bodies of the fish that eat them. The fish either die and wash ashore or become paralyzed and float to the surface. Birds eating those fish can die.
Next comes botulism poisoning.
"Zebra mussels have a connection with avian botulism," said Carrol Henderson, superintendent of non-game wildlife for the DNR. Botulism is a byproduct of mussel waste. The waste is eaten by fish, which then can infect fish-eating birds.
Thousands of birds in the eastern Great Lakes have died of botulism poisoning since 1999. Species again include loons, mergansers, grebes, cormorants, scoters and gulls.
Some species of diving ducks eat the mussels. Zebra mussels have been a dominant food consumed in the eastern Great Lakes by greater and lesser scaup and bufflehead ducks. Concentration of trace elements in the mussels kills the birds.
The scaup population fell from an estimated 7.5 million breeding birds in the 1970s to fewer than 4 million in 2005, according to author James H. Thorp. The figures come from his book "Ecology and Classification of North American Freshwater Invertebrates."
In Minnesota, long-tailed ducks also are divers that feed on mussels.
In Lake Mille Lacs, biologists have counted zebra mussel concentrations of 9,000 per square yard. If they were to colonize Lake Mille Lacs shore to shore, at that intensity, the total number of mussels there would be in the trillions. Those are big numbers for birds to be up against.