Cyanobacteria Bloom on Pleasant Lake
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SPECIAL ANNOUNCEMENT
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Cyanobacteria Bloom on Pleasant Lake
September 18 2025
There is a lakewide Cyanobacteria Harmful Algal Bloom (CyanoHAB) in Pleasant Lake and precautions are urged in terms of avoiding water contact. Drinking the water in particular can be lethal to small children and pets if these cyanobacteria are producing toxins. The bloom is of a toxin-producing variety, Dolichospermum, and the concentrations are close to a million cells per teaspoon (175,000 per mL). We do not know if they are producing toxins, and toxin production can be turned on and off in a short period of time. Toxins are often released at the highest levels when the cells die and disappear.
A dead cormorant was found in one of the worst bloom areas. It has been sent for a necropsy to see if the cyanobacteria caused the death. This seems unlikely, though we don’t know. See the information from the NH DES below, and the photos of the bloom from the DES (two of many that have been reported around the lake).
Note that wave action may mix the bloom down into the water so that it is less visible, or make it accumulate along the shoreline downwind. These blooms are complex and variable, but they have been reported on all sides of the lake in the past few days. You can get watch and warning emails from NH DES by signing up on their website here.
The Pleasant Lake Protective Association (PLPA) is working hard to understand and reduce the likelihood of future CyanoHABs in Pleasant Lake. We welcome volunteers, and it is important to realize that you, the residents and users of Pleasant Lake, are the lake managers. Membership in the PLPA and NH Lakes are good first steps toward helping curb CyanoHABs.
Prof. Craig Williamson, Resident Limnologist and PLPA Board Member
PLPA is managed by volunteers and supported by member donations.
If you wish to support our work, please consider becoming a PLPA member here.
