State officials revoked a water-quality warning earlier this week at Mascoma Lake after samples showed the concentration of cyanobacteria in the water dropped between 70 and 80 percent since they first posted the caution late last week.
A sample taken at Dartmouth Yacht Club on Tuesday contained 14 percent cyanobacteria -- well below the 50 percent state threshold for a warning. But a state expert said the reduced levels don't mean the cyanobacteria have disappeared, and officials will continue to take water samples from the lake each week.
"We know they didn't just die off," said Jody Connor of the New Hampshire Department of Environmental Services. "We know they're probably in there, they're just deeper in the water column right now."
Goose Pond in Canaan is now one of just three lakes or ponds in the Granite State with a warning in place after officials found that its levels of cyanobacteria, also known as blue-green algae, were still above the state threshold.
But the postings on lakes and ponds are cautions, not restrictions.
"If you notice anything resembling cyanobacteria, please refrain from wading, swimming, or drinking the water," the state's Web site said, adding that anyone who notices a blue-green substance near the lake surface should call the Department of Environmental Services at 603-419-9229.
Cyanobacteria, Connor said, changes depths in order to find the ideal conditions for feeding on phosphorous, which they need to grow and reproduce. When they want to produce chlorophyll, typically on sunny days, they can resurface.
"They're smarter than we are," he said.
In any lake or pond where cyanobacteria exist, even at a lower depth, Connor said people and animals shouldn't drink the water.
Lebanon draws its drinking water from the lake, but officials have ensured residents that the city's filtration system purges contaminants, making their water safe to drink.
Cyanobacteria contain toxins that can cause a variety of health problems, including liver damage, neurological effects such as loss of coordination or numbness, muscle pain, nausea, vomiting, diarrhea and skin irritation.
To get sick from cyanobacteria, a person must either drink water or inhale droplets containing the toxin, and people swimming in areas where cyanobacteria have bloomed can accidentally swallow small amounts of water, enough to make them ill. According to a report from the World Health Organization, evidence also shows that water-skiers or jet-skiers can get sick from inhaling contaminated spray.
Raymond Buskey, of the Mascoma Lake Association, said he's noticed fewer people frequenting the lake in recent days. He contributes the drop in visitors not only to the warnings, but also to reports that Dartmouth-Hitchcock Medical Center researchers are investigating a suspected link between cyanobacteria and clusters of ALS, or Lou Gehrig's disease, near Mascoma Lake and several other New England lakes and ponds.
"I think for a while it did deter people from putting their boats in and getting in the lake," he said. "Things just got extremely quiet very suddenly."
Buskey said he and other members of the association intend to encourage residents around the lake to stop using certain phosphorous-laden products -- such as fertilizer and specific dishwasher detergents -- that feed the cyanobacteria.
John Woodrow Cox can be reached at 603-727-3305 or jcox@vnews.com