A fisherman prepares to launch his boat at Cobbossee Lake in Monmouth in April 2022. An accumulation of algae on the water’s surface turned the lake green from mid-August through the end of September in what officials say is its worst algae bloom in a decade. Joe Phelan/Kennebec Journal file

Cobbossee Lake recently experienced its worst algae bloom since 2013, and experts are attributing it to this summer’s heavy rainfall.

Usually, algae are healthy for marine life. The microscopic organisms are an important part of a marine food chain, acting as a source of sustenance for smaller fish and other aquatic animals. Through photosynthesis, algae also release oxygen in the water. They are, in many ways, essential.

An algae bloom is seen recently on Cobbossee Lake. Heavy rainfall over the summer contributed to the worst bloom the lake has seen in a decade, officials say. Courtesy of Cobbosseecontee Lake Association

That is, until an abundance of them forms a green layer on the water’s surface, blocking oxygen and sunlight. This phenomenon, called an algae bloom, usually occurs due to an increase in nitrogen and phosphorus levels of a water body.

“That’s what happened. We had so much rain, and there was so much runoff going into the lake. That’s what caused the outbreak,” said John Stanek, a member of the Cobbosseecontee Lake Association.

Stanek added that there was a stark difference in the weather this year. In 2022, the water clarity of the lake was outstanding, he said, but that was due to a low level of runoff as there was barely any rainfall, and drought conditions mired the state. In 2023, the incessant rainfall led to runoffs across the region, bringing in pollutants in the form of soil, waste and fertilizers, all contributing to raising the lake’s phosphorus levels.

According to the Cobbossee Watershed District, a government entity working to maintain the water quality of the lake, one of the primary goals of water clarity is that it does not measure under 3 meters of visibility.


Cobbossee Lake did not meet this criterion from mid-August through late September. The last time this happened was in 2013.

According to Wendy Dennis, who studies freshwater systems as a limnologist with the watershed district, an algae bloom is expected to occur occasionally.

“The water quality of the lake is excellent at the beginning of each season, and it declines a little later into the season,” said Dennis. “The decline at Cobbossee this year, however, happened much earlier and was more dramatic than it has been in a decade.”

From 2013 through 2022, there have been four instances of algae blooms. In three of those instances, the bloom did not occur until mid-September and didn’t last long.

Only in 2013 did the bloom occur during summer, in mid-July, and like this year, it lasted more than a month. Therefore, even though the level of algae bloom did not meet the state’s criteria for a “severe” case, it gave the lake association a reason for close examination.

The good news is that most of the bloom has been cleared except for some remnants in the form of green scum along the lakeshores. The association had warned residents in late September to keep children and dogs away from these deposits as they can be toxic.


Dennis noted that the last round of on-water testing in late October showed that water quality level had improved and was just short of the 3-meter mark.

The roughly 5,500-acre lake crosses into several towns, including Winthrop, Monmouth, West Gardiner, Litchfield and Manchester.

Except for the algae bloom, Cobbossee Lake has had bouts with multiple milfoil infestations in recent years, presenting more challenges to the lake associations.

Moving forward, the associations are encouraging residents to take steps so that such occurrences can be avoided.

“We can’t control the weather, but what we plan to do is emphasize lake-smart living,” said Stanek. “Which is asking people to pay attention to their driveways and make sure they want to create a vegetative buffer to slow down run-offs.”

Additionally, lake associations are working to make available resources for buffer designs and educate people about not just milfoil but other problems like algae blooms. There are also grants that associations can apply for that can contribute toward road improvements to reduce runoff.

“This occurrence highlighted the fact that while the Cobbossee Lake water quality has improved a lot over the decades due to the pollution control work that has been done, it’s still somewhat fragile and vulnerable to pollution sources,” said Dennis.

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