July 6, 2017

Return of green slime threatens Greater Portland mudflats

Courtesy / Deb Dawson Photography.
Courtesy / Deb Dawson Photography.
An algae bloom is beginning to cover a large part of Back Cove in Portland.

How to report sightings of green slime

Friends of Casco Bay is keeping an eye on the green slime growth and is encouraging others to do so too. You can report excess green slime by emailing The organization encourages observers to take a photo and provide details about the location, with coordinates and landmarks, if possible.

For more information on nuisance algal blooms in Casco Bay go here.

Green slime caused by nuisance algal blooms has returned to Casco Bay's mudflats, threatening shellfish harvesting in the affected coves.

Friends of Casco Bay's staff, including Casco Baykeeper Ivy Frignoca, Research Associate Mike Doan and Intern Emily Haggett spotted bright green algal mats on Mill Cove and Antoine Creek in South Portland and Back Cove in Portland last week, according to a news release.

"These blooms can smother clams and other critters in and on the mud," Frignoca said. "We wondered if the drought conditions last year combined with nitrogen levels in the water caused the blooms, but this year, we have a very different weather pattern because of all the rain. We hoped the blooms last year were an anomaly, yet here we are observing, once again, the rise of slime."

This year's blooms appeared a week earlier than last year in Back Cove and weeks earlier on the South Portland side of the harbor, according to Friends of Casco Bay.

Doan said volunteer monitors also reported seeing phytoplankton blooms in Casco Bay since June 21.

"We are keeping an eye on the health of the bay," he said. "While phytoplankton blooms in water are expected this time of year, these are corresponding with the algae blooms on the mud flats — which we do not expect to see."

What causes the problem?

Excess nitrogen is one factor that encourages such blooms; a lack of sea life that might consume the algae could also be a factor. All living things need nitrogen to grow, but an overdose can trigger excessive growth of nuisance algae, reduce water clarity and lower oxygen levels.

Possible sources of nitrogen include natural sources, as well as man-made sources, such as air pollution, sewage overflows, lawn-care fertilizers and other components of stormwater pollution.

Friends of Casco Bay works with state and local government officials and volunteers to identify nitrogen levels and sources.


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