TURKEY POINT, Ontario— When a botulism outbreak hit this provincial park on Lake Erie in early September, it closed the beach. Thousands of dead fish, their white bellies glinting in the sun, washed up on the sandy shore. Hundreds of cormorants, gulls and terns fed on the dead sheephead, burbot and perch and quickly keeled over.

It was one of about two dozen sudden, random outbreaks this summer and fall on the shores of Lake Erie. Over the past four years dozens of similar outbreaks have occurred, all involving type E botulism, a rare strain of the potent nerve toxin. Experts say they still do not know what to make of the outbreaks.

Biologists are holding their breath, for over the next six weeks or so, migratory loons and the diving ducks called mergansers will make their way from Canada to the gulf coast for the winter. More than 8,000 common loons have been killed by the botulism, and the species is already in peril from overdevelopment. ''This is just another hit,'' said Dr. Grace McLaughlin, a wildlife disease specialist at the National Wildlife Health Center in Madison, Wis. ''It's pretty scary.''

The botulism causing the fish and bird kills is a rare type of the bacteria clostridium botulinum. Type C botulism, which also kills birds, is common throughout the country, notably in the Salton Sea in California; it occurs when drought dries up lakes, exposing invertebrates that harbor the toxin and are eaten by ducks and geese. Types A and B are the kind generally found found in poorly canned foods.

''I've seen botulism my whole career, but it's always been C,'' said Dr. Ward Stone, director of the New York State Department of Environmental Conservation's pathology laboratory in Delmar. ''When we found E, I was very surprised.''

Type E is unusual because it concentrates in fish, affecting a wider spectrum of wildlife than type C. It has been found in very small amounts in the Great Lakes for decades, but never for this long or in such large quantities. Something has happened recently to increase the abundance greatly, and biologists are trying to figure out what.

The prime suspects are three interlopers: the round goby, a small bottom-feeding fish; hard-shelled zebra mussels; and quagga mussels. All three species hitchhiked to the Great Lakes from the Black and Caspian Seas in the ballast water of international freighters -- the mussels in the past decade or so, the goby in just the past four years. The outbreak of botulism coincides with their widespread colonization of the lake.

Zebra and quagga mussels are thumbnail-size filter feeders that live on the bottom and filter phytoplankton, a one-celled alga, from the water. Paradoxically, they have made Lake Erie's water very clear, and that may be a cause of the botulism.

Clear water allows sunlight to penetrate to greater depths, hastening the growth of aquatic plants. Botulism flourishes in the sort of oxygen-deprived environment provided by decaying plant matter.

Scientists theorize that mussels may also be biological magnifiers, concentrating the toxin as they filter water to get nutrients or excreting the toxin into the mud around them. When the mussels are eaten by gobies, a species of bottom-feeding scavenger, the botulism is concentrated further. Crippled by the toxin, the gobies thrash about in the water and attract predatory fish, mud puppies (a kind of salamander), mergansers, loons and other birds.

Dead or dying fish on the lakeshore are an ideal environment for botulism. They attract scavengers like gulls and terns -- and, in one case, a bald eagle -- that die when they eat the contaminated fish. Maggots from eggs laid on the fish become contaminated and pass the toxin along to birds that eat them. Mergansers and loons ingest the toxin by eating live fish.

Another contributor to the botulism outbreak may be warming temperatures in the relatively shallow lake, whose average depth is 60 feet. Last year was one of the rare winters when Lake Erie did not freeze over.

Botulism, an extremely potent and fast-acting toxin, blocks the transmission of impulses from nerves to muscles. (Type A botulism is the active ingredient in Botox, the drug that is injected in fashionable foreheads to paralyze the frown lines of aging.) Birds that ingest botulism lose their ability to fly and even hold their heads up -- the condition is called limberneck -- and often die by drowning. Fish can no longer swim.

Humans are not considered to be at great risk, but officials have warned anglers not to eat fish that are found floating or that do not fight on the end of the line. Thorough cooking kills botulism, but it may survive in some smoked fish. In July, 14 Alaskan native villagers became seriously ill from type E botulism when they ate the tail of a beluga whale that had washed up on a Pacific beach.

Botulism is not the only problem associated with the exotic species, which have taken over Lake Erie with astonishing speed. Zebra mussels showed up in 1987 and in a few years covered hard surfaces throughout the Great Lakes, from sewer pipes to rocks. Quaggas showed up a decade ago, and they now outnumber zebra mussels 10 to 1 because they also fasten to muddy and sandy bottoms.