DURHAM, N.H. – A University of New Hampshire plant biology professor has documented an abundance of invasive species of seaweed in Maine’s Casco Bay that appear to be changing the ecosystem.

Arthur Mathieson recently completed a five-year project in which he collected more than 8,500 species of seaweed from 200 sites in Casco Bay, which stretches from Cape Elizabeth to Phippsburg.

He compared his list with the seaweed collection of Frank Collins, a self-taught botanist who collected specimens off the Maine and New England coasts a century ago.

The old collection, with historical diaries that Collins kept, has been split up among several universities and botanical centers.

Mathieson’s conclusion is that there has been a 25 percent change in the species mix in the bay between a century ago and now. He hopes his collection will help scientists in the future.

“I enjoy the ocean,” Mathieson said. “I’d like to do something to help protect it.”

For his research, which involved students and several colleagues, Mathieson visited the sites during all four seasons. He dried and flattened the seaweed specimens, attached them to pieces of paper and wrote their Latin names on the bottom. They are now stored at the Hodgdon Herbarium on campus.

To illustrate his findings, he displayed a sample of a Codium fragile, an invasive species that is native to Japan. This variety, he said, is an example of a harmful seaweed that can grow rapidly and change its habitat.

Many species make their way to the United States by boats from Asia. The seaweed can attach to a ship or be released with ballast water from the ships’ hulls, he said.

While many new species have made their way to Casco Bay waters, others are dying off, he said. The change in 100 years is a “significant loss,” he said.

And the new species can have a detrimental impact. Codium fragile, for instance, can lift shellfish and carry them out of their natural habitat, or grow over the two halves of a shell and keep it from opening to feed.

New species often don’t have natural predators in their new environment, he said. And if warming trends continue, he expects to see more warm-water species making their way to Maine waters.

Mathieson’s project was funded by the National Oceanic and Atmospheric Administration and New Hampshire Sea Grant.