Tackling a ‘puzzling’ problem on Pleasant Lake

NEW LONDON — Craig Williamson, of Elkins, has been spending time at his family home on Pleasant Lake since 1959.

The 600-acre lake is “one of the last where I would ever expect to see cyanobacteria blooms,” Williamson, who has also spent decades as a scientist studying inland aquatic ecosystems like lakes, said in a recent interview.

But in June 2022, Pleasant Lake saw its first recorded cyanobacteria bloom, according to locals and New Hampshire Department of Environmental Services data.

Pleasant Lake has seen a similar bloom every summer since.

The trend has prompted the Pleasant Lake Protective Association (PLPA) to ramp up water quality monitoring this season and try to answer the big question: What is causing the annual blooms that can release toxins and prompt health effects including skin irritation, vomiting and neurological or liver damage?

Pleasant Lake experienced a bloom of the naturally occurring bacteria from June 30 through July 8 this year, limiting lake use during some of the busiest days of the summer.

“It has been a mystery as our lake water quality is one of the best in the state,” said PLPA President Doug Baxter. “Yet, we have had these puzzling blooms occur.”

In 2023, New Hampshire’s Department of Environmental Services established a statewide plan to address the blooms because of their increasing frequency in New Hampshire’s waters.

“Cyanobacteria blooms are significantly impacting New Hampshire’s surface waters,” the plan reads. “An increase in documented bloom occurrence and severity requires action to reduce and control impacts related to public health, recreational and commercial water uses and the economy supported by the state’s surface waters.”

From 2017 to 2024, New Hampshire cyanobacteria warnings lasted 23 days on average, according to data from the Department of Environmental Services. Since 2020, blooms have been reported on more than 60 waterbodies every year, a number that has been steadily climbing since 2015.

In 2026, there have been 28 cyanobacteria reports issued across 26 lakes as of Thursday, according to data from the department, including on Pleasant Lake and Enfield’s Mascoma Lake.

An unlikely place for a bloom

While not an uncommon problem, the qualities of Pleasant Lake make the blooms particularly curious, Williamson, Chairman of the PLPA Watershed Management Committee, said.

Pleasant Lake stretches more than 90 feet at its deepest point and is classified as oligotrophic, meaning that it has clear water that is low in nutrients — factors that should minimize the risk of dangerous blooms.

“Because it’s so deep it can absorb a lot of abuse around the edges,” Williamson explained. “The problem is that if we continue that, eventually you reach a tipping point and then it’s all over. It’s very, very hard to go back.”

The lake could become unhealthy and unpleasant for swimmers and wildlife.

“If we reach that tipping point, it’s going to be pretty ugly,” Williamson said.

Pleasant Lake and the surrounding watershed are home to a range of wildlife, including a pair of much-beloved nesting loons and their chicks, other waterfowl like common mergansers and Canada geese, bald eagles, painted turtles, bass, pickerel, beavers, dragonflies and damselflies. The PLPA even releases a regular newsletter focused on the lake’s wildlife, called The Nature of Pleasant Lake.

Historically, the lake was a “really quiet, serene place,” Williamson said, though increased activity, including more motorboats, has started to change that.

While the cause of the blooms is unknown, Williamson expects that they are being bolstered by human activity around the lake.

Factors like fertilizers from nearby lawns and septic system leaks bring bloom-causing nutrients like phosphorus into the water, while warming lakes make for a more hospitable environment for the bacteria.

For the 2026 season, the protective association has introduced “aggressive testing measures” for the lake in addition to a pre-existing monthly water testing program, Baxter said.

Shining a light and gathering data

A neon-yellow solar-powered buoy is constantly analyzing water quality at one end of the lake.

The buoy, installed in June, samples water at two depths and measures parameters including temperature, dissolved oxygen, chlorophyll and a pigment produced by cyanobacteria.

The data is recorded in real time and posted to an online dashboard along with local weather data.

The installation from a Woods Hole, Mass.-based research company called Subtidal is part of a pilot project for the company. Pleasant Lake is one of 12 across New Hampshire, Massachusetts and Maine where Subtidal is analyzing water quality in 2026, CEO Casey Wilson said Wednesday.

The monitoring buoys are designed to help address two big questions: is the water safe and what are the root causes of cyanobacteria blooms on each lake?

At the end of the June to November monitoring season, Subtidal’s scientists analyze the data and write a report on their findings and possible solutions for each lake. Wilson expects that lake associations could use the data to help secure funding for expensive remediation projects.

“It can be a really effective advocacy tool to then get the interventions that will be helpful for addressing those issues,” Wilson said.

So far, there have not been any “smoking guns” to indicate what is causing Pleasant Lake’s cyanobacteria problem, Wilson said.

But the data has started to reveal some trends. For example, cyanobacteria levels are low at the lake’s surface and much higher at the bottom, indicating that they might be building up there and rising, Wilson explained.

The cyanobacteria bloom in late June was not picked up by the buoy because it was at the other end of the lake. Wilson said the working theory is that the bloom was caused by cyanobacteria rising from the bottom of the lake and being blown to shore.

Subtidal is charging about $1,000 per month through the monitoring season for the buoy, real-time data and an end-of-season assessment, Wilson said. The company has worked hard to bring down the price of the program so it is accessible for lake associations.

An anonymous community member is paying for the Pleasant Lake buoy project, Williamson said.

Volunteers step in

PLPA volunteers are also manually collecting water samples throughout the season at different locations.

Some are collected weekly and sampled for potentially toxic cyanobacteria levels. Others are taken monthly to be analyzed for toxic species and the levels of microscopic plankton that feed on cyanobacteria, Williamson said.

The new regimen complements a testing program the PLPA has run for years through the New Hampshire Volunteer Lake Assessment Program.

Volunteers head onto the lake once a month and sample water at the deepest point and other locations like where tributaries enter the lake. The water samples are tested for parameters including the temperature, oxygen and chlorophyll levels that the Subtidal buoy is also logging.

Williamson hopes that the increased water testing will help fill in knowledge gaps by collecting constant data, testing for different parameters, and sampling water at different locations around the lake.

Historically, cyanobacteria blooms have only been recorded when people happen to see them and water has only been tested in certain locations on a monthly basis. There are likely cyanobacteria blooms that are never seen.

Getting the word out

In addition to the increased testing, the Protective Association is hoping to raise awareness about water quality and engage residents as “lake stewards,” said Valerie Burns, Williamson’s wife and a PLPA member who recently started a water-quality-focused newsletter.

“We have not had a sense of community here at Pleasant Lake because people live in their silos,” Burns said.

She launched the newsletter, called Ripples, in April and plans to focus not on the “negatives” like “what people are doing that is harmful and selfish,” but on positive actions to help build community.

The newsletter includes sections that address water quality monitoring efforts, and offer advice for locals like what native plants they might consider growing in their gardens to protect water quality and prevent shoreline erosion.

“There’s a lot of eye-opening that we are looking to do so people understand if you live in the watershed, the watershed is what feeds the lake,” Burns explained.

Burns said she hopes that people who live around the lake and other nearby lake associations will collaborate on a solution.

New London is one of the four towns bordering Lake Sunapee that has adopted an ordinance requiring residents to maintain and pump their septic systems to avoid discharges into the lake. Enfield has its own similar septic ordinance designed to protect Mascoma Lake.

The Protective Association has also applied for a $100,000 loan from New Hampshire’s Clean Water State Revolving Fund to put together a Watershed Management Plan.

The plan will use scientific surveys to determine how much phosphorus is entering the lake, where it is coming from and outline projects that might mitigate the flow. Lake Sunapee has its own Watershed Management Plan effective through 2030.

“We’re just trying to do our part to counter some of these activities,” Burns explained.