TORONTO - The health of the Great Lakes ecosystem is improving in some ways, but the impacts of population growth, climate change and invasive species continue to raise troubling concerns about the future of the waters, says a new report by Environment Canada and the U.S. Environmental Protection Agency.

The report diagnoses the Great Lakes' well-being as "mixed'' since some problems are improving while others are getting worse, said Environment Canada's Nancy Stadler-Salt, who managed the team that provided the report's Canadian content.

While the Great Lakes continue to be a good source for treated drinking water, and levels of toxic chemicals have been significantly reduced over the last 30 years, there are some problems that can't be fixed, she said.

The report says more than 300 invasive or non-native species now thrive in the Great Lakes basin -- and their destructive, parasitic behaviours are difficult to contain.

"That's a huge concern, (especially) how they compete with our native species and in some cases, may replace them or drive them out of certain habitats,'' Stadler-Salt said Monday.

"And once they're here, they're probably next to impossible to eradicate -- it's just learning how to control them.''

Many invasive species get into the Great Lakes basin from the ballast water of cargo ships from all over the world.

The Great Lakes region is a major trade centre, so keeping the ships at bay is simply not an option, Stadler-Salt said, even though the damage caused by invasive species is estimated to cost billions of dollars annually.

While research is ongoing to find a way to treat ballast water to stop the spread of invasive species, there's still no solution, she said.

The report also suggests it's virtually a given that climate change will have an impact on the Great Lakes region.

"All the impacts aren't known, but we know that storms are going to get more severe,'' Stadler-Salt said. "We may have shorter winters and less ice coverage on the lakes in the winter.''

An altered climate and lower water levels would also affect wetlands and continue to force wildlife from their natural habitats.

Maude Barlow of the Council of Canadians said that could have a disastrous effect on the lakes' ecosystem and must be addressed.

"They say wetlands are the kidneys of our waterways and forests are the lungs, and when you allow the destruction of wetlands to the extent that has been allowed to happen around the Great Lakes ... there's a potential for real disaster,'' she said.

"The Great Lakes are a potential tragedy waiting to happen, or maybe it's already happening.''