A report released Thursday from the nonprofit Lake Winnipesaukee Alliance finds that the lake’s water quality has been steadily decreasing, according to data stretching back more than 40 years.
The group said the comprehensive report provides scientists a baseline for monitoring the lake's future ecological health and offers concrete ways to better protect the lake.
Both increased development and climate change are contributing to worsening water quality on the lake. Eric Siy, the Alliance’s executive director, said development leads to more damaging runoff flowing into the lake.
“Roads, driveways, rooftops [lead to]... water from rain events that is literally rushing off and directly into the lake rather than soaking into the ground and filtering through and and much at a much slower pace.”
Siy said three main factors have diminished water quality: stormwater runoff, septic effluent and road salt. Stormwater and septic leaching can bring phosphorus into the lake, and road salt triggers the release of phosphorus from the lakebed. Phosphorus serves as a nutrient that spurs algae and cyanobacteria growth, disrupting ecosystems. Even small amounts can impact aquatic ecosystems significantly: Just one pound of phosphorus can promote the growth of 500 pounds of algae.
The report says phosphorus levels are predicted to increase by nearly 40% in coming decades.
Increased development around the lake means more homes and paved roads than a few decades ago. That means fewer surfaces to absorb stormwater, more septic systems that leach phosphorous and more winter drivers who want their roads covered in salt.
Over the past seven years, harmful cyanobacteria blooms have been increasing substantially, by an average of 23% per year.
Siy said cyanobacteria can turn clear blue water to something that looks more like pea soup.
“Water clarity has declined more than eight feet over the last 40 plus years,” Siy said. That means the depth at which scientists can clearly see a disc lowered into the water has decreased by eight feet.
But, Siy said, the changes to the lake water are still reversible, if communities take action. That could include better capturing storm water before it runs into the lake, using road salt more judiciously and improving out-of-date septic systems.
“We are not trying to land anybody on Mars or, or even the moon. We are simply putting Lake Winnipesaukee on a lower nutrient, low-salt diet,” he said.
Climate change presents a formidable challenge, he acknowledged, calling it “an accelerant” of some of these processes.
“We are seeing shorter winters, longer growing seasons, warming waters,” Siy said. “So all of that contributes to a faster pace of algae growth.”
Cyanobacteria flourished in the lake in 2024, the hottest summer ever recorded in the state, keeping many people from safely swimming. As climate change makes the state warmer and wetter, this presents a growing concern for lakes across the state.
Siy also said invasive species are a threat to Winnipesaukee’s ecosystem.
“We still only have four known aquatic invasive species. Lake Champlain, by contrast, has 50 plus invasives. It's a nightmare scenario,” he said.
Siy said trailered boats are the most common way invasive aquatic species get into the water. He said people need to be vigilant about inspecting their boats and taking necessary precautions before launching them into the water.
Siy encouraged people who care about the lake to learn more about how they can help protect it for future generations.
“The cavalry is not coming. We are it,” he said. “We being everyone who knows and loves and depends on this lake being clean and clear.”
The state also sued three construction companies for alleged unlawful dredging that it claims hurt water quality, the Department of Justice announced Thursday.
