Cyanobacteria continues to vex Lake Kanasatka

MOULTONBOROUGH — The state’s Department of Environmental Services has issued a cyanobacteria warning for Lake Kanasatka, a lake which was treated last year to prevent such a condition from returning.

The warning, which remained in effect as of Wednesday evening, was issued on June 10, based on water samples tested on June 8, after observers saw green and yellow ribbons forming in the water near the shoreline.

Tests of the water found cyanobacteria at a density of 600,000 per milliliter, about nine times the concentration threshold of 70,000/mL, which the state uses to gauge whether a bloom is serious enough to trigger a warning.

DES staff advise against swimming in water with a present cyanobacteria bloom. Exposure to toxins created by the bacteria can cause acute conditions in humans such as irritation of the skin and eyes, as well as tingling or numbness, nausea, vomiting, diarrhea and seizures. Long-term exposure can cause damage to kidneys and the nervous system. The toxins are also known to be harmful to animals, such as livestock and pets.

Lake Kanasatka has been plagued by cyanobacteria blooms. The algae, though naturally present in the environment, can be spurred into harmful blooms by the disruption of the water body’s natural chemical balance. Too much phosphorus, to be specific, is typically the cause, and the nutrient commonly finds its way into the water through careless use of fertilizers, road runoff or through failing septic systems.

In addition to this current advisory, DES has issued nine other cyanobacteria warnings for Kanasatka since 2020, and just as many watches, which are issued when there’s visual evidence of a bloom but no sample taken, or if the concentration tested was not high enough to trigger a warning. Kanasatka is the only lake in the state with a current cyanobacteria watch.

This year’s bloom is notable in that it’s the first since the lake received an unusual treatment to prevent cyanobacteria. An association of property owners determined that phosphorus was being stored in the lake bed and released into the water during the height of summer. DES leaders agreed with their assessment and approved the use of aluminum sulfate, which binds with the phosphorus and keeps it out of reach from cyanobacteria.

To stay up-to-date with swimming advisories, visit des.nh.gov/water/healthy-swimming/healthy-swimming-mapper.