AMESBURY — A second bloom of dangerous blue-green algae has been detected in Lake Attitash, and residents are once again being advised to stay out of the water until the bloom runs its course.
The Lake Attitash Association announced that a water sample taken at the state boat ramp in Merrimac earlier this week found dangerously high concentrations of the algae in the lake’s waters, prompting the state Department of Public Health to issue an advisory urging residents to steer clear of the water for the time being.
Blue-green algae, which is known scientifically as cyanobacteria, can produce toxins that can be harmful to humans and pets in high enough concentrations, and the frequent blooms on Lake Attitash have been a persistent thorn in the side of local residents and officials over the past few summers.
This is the second time this season that the state has issued a blue-green algae advisory for Lake Attitash. The first was issued on June 17 after similar blooms were detected in the lake, and it was later lifted on July 5 after two consecutive “clean” samples were taken from the water.
The state has three criteria to determine if dangerous concentrations of cyanobacteria are present. One is if a water sample contains over 70,000 cyanobacteria cells per milliliter; another is if the water contains microcystin toxins above 14 parts per billion; and the last is if visible patches of scum can be seen on the water.
The latest sample taken on Monday contained 100,000 cyanobacteria cells per milliliter, which is well over the state guideline. The state collects water samples weekly, meaning that the earliest the current advisory can be lifted is July 31, assuming the next two both come back clean.
While a state advisory doesn’t prompt a closure of the lake, the Lake Attitash Association has urged residents to obey the advisory and stay out of the water when blue-green algae blooms are present.