Stresses from polluted rivers to invasive species threaten to trigger an ecological breakdown in the Great Lakes, a group of scientists hoping to sway U.S. environmental policy said Thursday.

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Seventy-five scientists who study the world's largest collective body of fresh water released their report on the myriad problems that need cleanup or restoration ahead of two key policy announcements next week.

"This is just a critical period for the Great Lakes," Andy Buchsbaum, director of the National Wildlife Federation's Great Lakes office, said about next week's announcements.

A task force comprising federal agencies, Congress, local government officials and regional Indian tribes is scheduled to release its much-anticipated final plan for preserving the Great Lakes requested by President Bush in 2004.

The body's preliminary report in July recommended $20 billion in federal, state and private funding over 15 years to upgrade antiquated municipal sewer systems, restore 500,000 acres of wetlands, clean polluted harbors and bays, and pay for other efforts.

But a federal oversight group subsequently suggested to the White House that the budget was too tight to allow additional funding. Federal spending on Great Lakes cleanup over the past decade was $800 million, according to the Government Accountability Office.

After the task force releases its plan Monday, governors representing U.S. states and Canadian provinces that border the Great Lakes will announce revisions to century-old rules that restrict water withdrawals and diversions from the lakes.

More than 30 million people rely on the Great Lakes for drinking water, and large-scale diversions to far-off states or countries have been forbidden.

Threats to the Great Lakes are converging, scientists who worked on the report said.

"There's widespread agreement that the Great Lakes are under tremendous stress," said Alfred Beeton of the University of Michigan. "Toxic substances ... overfishing, invasive species, changes in hydrology affecting rivers -- now we can add the effects of global climate change.

"These have been dealt with individually. What we need to do is look at the ecosystem -- the combination of stresses," Beeton said. "Historical sources of stress have combined with new ones and we have arrived at a tipping point. What we mean is that ecosystem changes will occur rapidly and unexpectedly."

The report emphasized the need for large-scale ecosystem restoration and not piecemeal efforts, coauthor Don Scavia said. Particularly important was preserving or restoring shoreline "buffer zones," such as wetlands and lake tributaries to help the lakes heal themselves.

"These are the key areas for filtering the contaminants that enter the lakes. It's also where most of the wildlife habitat is," Scavia said.

Shoreline pollution that fouls Great Lakes beaches is extending into the middle of some of the five Great Lakes, sudden drops in oxygen levels in the water threaten native species, and native fish have been crowded out by invasive species that have changed the character of the lakes, the scientists added.