Come spring, Highland Lake will be busy, and not just with boating and fishing.

Beginning in May, researchers will launch extensive testing of water quality in the lake on the Windham-Falmouth town line. Their goal is to determine the cause of a bloom of blue-green algae that has appeared every July for the last four summers. The mysterious bloom has prompted the towns to study their zoning ordinances and has disrupted development in the watershed.

The nonprofit Highland Lake Association and other parties will host a public forum March 21 about the health of the lake and the triage conducted so far. “The lake you love is in trouble,” said Rosie Hartzler, a Windham resident and the president of the lake association. “It’s going to take the whole community to solve this problem.”

A popular destination for recreation, Highland Lake is only 7 miles from downtown Portland.

In the 1990s, the water was so murky that it was deemed an “impaired lake” by both the Maine Department of Environmental Protection and the federal Environmental Protection Agency. With nearly $1 million in grants, the help of the Cumberland County Soil and Water Conservation District, and the efforts of local residents, it was removed from that list in 2010.

The strange bloom first appeared in late July 2014. The little particles that make up the bloom are called picocyanobacteria, and they disperse in the water in a way that has not been observed in any other lake in Maine, at least so far. There is no evidence that this bloom is toxic, but it causes a pervasive and noticeable cloudiness in the water.

No one knows why this bloom happens like clockwork every summer. Hartzler and others believe that it is caused or at least accentuated by an increasing amount of phosphorous in the lake. Phosphorous, one of the major nutrients needed for plant growth, is found in natural soils and commercial lawn fertilizers. When soils or fertilizers get washed into the lake by storm runoff, the phosphorus effectively fertilizes the lake and can cause algae to multiply into a visible bloom. Development in a watershed can increase runoff from new lawns, rooftops and roads.

Karen Wilson, an associate research professor in the Department of Environmental Science and Policy at the University of Southern Maine, will be involved in much of the testing that will take place this summer. Wilson said that while it’s still unclear what is causing this particular bloom, the future health of the lake depends on controlling its phosphorous levels now.

“No matter what we find as the immediate cause of the bloom, it’s going to be incredibly important to control nutrients coming into that watershed,” Wilson said.

Out of concern about phosphorous runoff, the Highland Lake Association successfully lobbied the Windham Town Council in September to pass a six-month moratorium on construction near the 623-acre lake. Last week, the council extended that ban for up to another 180 days.

The moratorium was not popular with all homeowners and property owners in the watershed. The decision halted projects ranging from garage add-ons to subdivisions. One developer, John Chase of Chase Custom Homes, had applied to build 10 rental townhouses and 10 storage units, as well as a gated community with 24 single-family homes. After the moratorium vote, he threatened legal action. Hartzler said she believes Chase is willing to change his plan, but Town Manager Tony Plante said he has not withdrawn his application or made any amendments to it. Chase did not respond to a request for comment for this story.

During the moratorium, the lake association convened a roundtable of 15 scientists and officials to discuss their hypotheses on the bloom. In May, that group will undertake a watershed survey, which will identify sources of runoff. It has applied for a $12,000 watershed protection grant from the town of Windham, which would be matching funds for mitigation projects in the worst areas around the lake.

“We’re very serious about implementing strategies this summer” to reduce that runoff, Hartzler said.

The town councils in Windham and Falmouth agreed to form a joint committee to review their ordinances and plan for the lake’s future. State law already requires shoreland zoning to protect water bodies in Maine, but officials in Windham proposed a series of ordinance changes that would, among other things, set a stricter standard for phosphorous runoff. No decisions have been made yet, but town managers in both Windham and Falmouth said they anticipate new rules in the coming months.

“What’s important about this forum is that it gives the public an opportunity to hear and understand what we know so far,” Plante said. “(Lakes and ponds) are a big part of the quality of life in Windham. If we don’t protect them, we’ll lose them.”

The public forum will take place at 7 p.m. on March 21 at the Windham High School auditorium.

Megan Doyle can be contacted at 791-6327 or at:

mdoyle@pressherald.com

Twitter: megan_e_doyle