State issues cyanobacteria warning for Silver Lake

CONCORD – An elevated cyanobacteria cell concentration has been measured in Hollis’ Silver Lake. Samples revealed that the state threshold of cyanobacteria was exceeded. The cyanobacteria were identified as Anabaena sp. As a result, the New Hampshire Department of Environmental Services has issued a cyanobacteria warning for those who recreate on Silver Lake. The current bloom is variable with some lake portions free of scum sightings, while other areas have small blue-green clumps in the water. The heaviest bloom areas were observed near the dam. Please continue to monitor your individual shoreline for changing conditions.

This warning is not based on a toxin evaluation and is intended as a precautionary measure for short term exposure. DES advises lake users to avoid contact with the water in areas experiencing elevated cyanobacteria cell conditions typically where lake water has a surface scum or blue-green flecks. DES also advises pet owners to keep their pets out of any waters that have a surface scum or blue-green or bright green flecks.

DES routinely monitors public beaches and public waters of the state for cyanobacteria. Once a cyanobacteria warning has been issued, DES returns to affected waterbodies on a weekly basis until the cyanobacteria standards are again met. Cyanobacteria are natural components of water bodies worldwide, but blooms and surface scums may form when excess phosphorus is available to the water. Some cyanobacteria produce toxins that are stored within the cells, but released upon cell death. Toxins can cause both acute and chronic health effects that range in severity. Acute health effects include irritation of skin and mucous membranes, nausea, vomiting and diarrhea. Chronic effects include liver and central nervous system damage.

The warning went into effect on June 12 and will remain in effect until additional samples reveal cyanobacteria levels have diminished.

Visit the DES Beach Program website for photos and more information about cyanobacteria at

Updates on advisories and warnings may be obtained at

If you notice anything resembling cyanobacteria, please refrain from wading, swimming, or drinking the water. Keep all pets out of the water and contact DES immediately. Call DES to report a cyanobacteria bloom at 848-8094.