WINDHAM — Since 2014, Highland Lake has seen a temporary but concerning drop in water clarity for several weeks every July and August.

The Highland Lake Association is hard at work to understand why.

“The entire lake gets a tint to it,” says Tom Bannen, the association’s president and a Falmouth resident.

That temporary tint, the group is increasingly confident, is caused by a bloom of cyanobacteria – also known as blue-green algae.

Though the likely culprit has been identified, questions about its origin and how to address it remain.

Bannen said there are no indications that anyone has suffered ill health effects from the cyanobacteria, even during peak bloom, but the situation still has the association looking into the lake’s health.

“We need to figure out why this cyanobacteria is proliferating,” said association Vice President Rosie Hartzler, who lives by the lake in Windham.

While the association is confident that the cyanobacteria is caused at least in part by an excess of nutrients such as phosphorous and nitrogen that create an imbalance in the lake’s food chain, what they are still wrestling with is where those nutrients are coming from.

Bannen speculated that there could be multiple factors, including temperatures, early ice-out dates, people disturbing soil around the lake more than they used to, and more lakeside development.

Complicating things further, the association says the lake is the only one in Maine they know of that has experienced this particular bloom, meaning there’s no playbook to follow in addressing the issue.

“No one else has seen what Highland Lake has seen,” Bannen said. “We’re in a unique situation.”

Highland Lake stretches more than 600 acres in Falmouth, Windham and Westbrook and is just less than 70 feet at its deepest point.

From the late 1990s until 2010, the lake had been designated by the state as at risk from over-development. When the lake showed enough improvement, it was removed from that list.

“I think everybody just took a bit of a collective sigh,” Hartzler said about the improvement in the lake’s status.

But four years later during a routine water quality sampling, the lake association received an unpleasant surprise.

Water testers use a device called a Secchi disk to measure clarity. The black-and-white disk is lowered into the water until it can no longer be seen, and the depth at which it goes out of sight is recorded.

During testing in 2014, testers saw a dramatic decline in clarity. While Bannen said it’s not unusual for Highland Lake to be in the 5-meter range, the surprise readings in 2014 were under 3 meters. They have also seen readings as low as 1.7 meters during the bloom.

The lake association has created a nine-member subcommittee focusing specifically on the bloom. The panel is composed of members from both the association and the community at large.

Bannen and three individuals are certified by the Maine Department of Environmental Protection to conduct water sampling, and they are taking weekly readings and samples from the lake this summer at the deepest part of the lake.

They are stepping up the monitoring with a focus on the root causes of the cyanobacteria.

During a testing in early June, Bannen said they were encouraged by the clarity reading of 7.3 meters that they recorded.

The reading was “one of the best we’ve ever had,” said Keith Williams, who has spent more than three decades monitoring the health of Highland Lake.

Williams said that they’ve had to reconsider what they thought they knew in addressing the bloom at the lake.

“Science is a self-correcting activity,” said Williams, an Army veteran with four degrees, including a doctorate in environmental science from the University of Oklahoma.

The lake association is also connecting with researchers at from a variety of institutions, including the University of Southern Maine, University of New Hampshire, Plymouth State University and Bigelow Laboratory for Ocean Sciences. The researchers are helping to analyze the samplers from the lake to learn more about the cyanobacteria and nutrients in the lake.

All of that testing and analysis isn’t free, and the lake association has received a $4,000 grant from the town of Windham to support those efforts and the lake association’s efforts to raise awareness about the situation.

The group is also making a big push to get the word out at its upcoming annual meeting at the Cornerstone Assembly of God church on the evening of July 20.

Bannen says that all of the time and energy dedicated to understating the water quality situation is worth it.

“We’re all busy in our lives,” he said, “but this is important.”

Matt Junker can be reached at 781-3661 ext. 123 or [email protected]. Follow him on Twitter: @MattJunker.

Highland Lake from the shore in Falmouth.Keith Williams takes readings during water testing and sampling at Highland Lake last month.