Dozens of volunteers turned out Sunday morning to collect water samples from Casco Bay in an effort to capture a snapshot of nitrogen levels.

The volunteers took water samples along the Fore River and Portland Harbor from boats, docks and mucky shorelines in a project organized by the Friends of Casco Bay, a South Portland-based nonprofit that works to improve and protect the bay’s environmental health.

Twenty-five years of water sampling has shown that high nitrogen levels are changing the chemistry of the bay, triggering algae blooms, fish kills and brown foam, according to the group.

“It is a local problem with a local solution,” said Casco Baykeeper Ivy Frignoca, an environmental lawyer who acts as the lead advocate for the Friends of Casco Bay.

High nitrogen levels are caused by sewer system overflows, atmospheric emissions from vehicles and industrial sources, and runoff from farms, lawns and city streets. Friends of Casco Bay has headed a long-term campaign to get homeowners to stop using lawn fertilizers, a major source of nitrogen in runoff.

Past sampling has shown that nitrogen levels are highest near river mouths, shorelines and sewer overflow pipes. The water quality improves farther offshore.

The idea for Sunday’s sampling, dubbed Nabbing Nitrogen, was to try to pinpoint sources of nitrogen in Portland Harbor with the goal of finding more ways to reduce its introduction into the bay.

Volunteers took water samples from about 80 sites, at low tide, precisely at 10:10 a.m. Plans for sampling another 70 sites by volunteers in kayaks and other boats had to be scrapped because of rainy, windy conditions. A staff scientist was able to collect some samples by boat.

The results were frozen within an hour of collection to ensure they remain fresh. They will be sent to a laboratory for analysis, which will take about a month. Results will be announced in the coming months. The city of South Portland and the Maine Department of Environmental Protection are taking part in the project.

Volunteers turned out in force Sunday, and there had been a waiting list for taking part.

“It’s incredible how many people love Casco Bay,” Frignoca said.

Mac Richardson of Windham said he volunteered because it is up to the general public to solve the problem, by reducing the use of lawn fertilizers and cleaning up pet waste.

“I love Maine’s rivers. They are such a blessing to everyone. So anything I can do to make our waters better is something I want to do,” Richardson said.

Members of Portland Public Works’ water resources staff also volunteered.

“This is our way of joining hands and being a good partner,” said Nancy Gallinaro, water resources manager.