State Issues Cyanobacteria Bloom Advisory for Silver Lake in Hollis, NH

Concord, NH - Cyanobacteria activity has been observed on Silver Lake in Hollis, N.H. A white surface scum was reported on Friday 8/5/22, a sample reviewed then had moderate levels of several types of bloom forming cyanobacteria (Woronichinia, Microcystis), but was not above the advisory level. Samples were collected again today, 8/8/22. While the white surface scum had subsided, there were floating aggregations of brown green mats. A sample of this concentrated material had a density of 329,000 cells/mL cyanobacteria (Planktothrix) with additional aggregations of cyanobacteria (Oscillatoria and Spirulina) too numerous to count. Advisories are issued when cyanobacteria cell concentrations exceed 70,000 cells/mL. As a result, NHDES has issued a cyanobacteria bloom advisory for those who use the waterbody for recreation. The advisory is not based on a toxin evaluation and is intended as a precautionary measure for short term exposure.

  
Image on left from Friday 8/5/22 morning; image on right from Monday 8/8/22.  

Surface blooms can rapidly change and accumulate in various locations around a waterbody. Please continue to monitor your individual shorelines for changing conditions. NHDES advises lake users to avoid contact with the water in areas experiencing elevated cyanobacteria cell conditions, also known as a bloom. NHDES also advises pet owners to keep their pets out of any waters that have a cyanobacteria bloom.

Cyanobacteria are natural components of water bodies worldwide, though blooms and surface scums may form when excess nutrients are available to the water.  Some cyanobacteria produce toxins that are stored within the cells and 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, tingling, numbness, nausea, vomiting, seizures and diarrhea. Chronic effects may include liver and central nervous system damage. Be cautious of lake water that has a surface scum, changes colors, or appears to have green streaks or blue-green flecks aggregating along the shore.  

The cyanobacteria advisory went into effect on August 8, 2022, and will remain in effect until NHDES confirms that cell concentrations of the bloom have subsided.

• Visit the NHDES Beach Program website for photos and more information about cyanobacteria at Harmful Algal Blooms.
• Updates on cyanobacteria advisories may be obtained on the Beach Advisory Mapper.
• Follow the Beaches twitter feed.

If you notice anything resembling cyanobacteria, please refrain from wading, swimming or drinking the water. Keep all pets out of the water and contact NHDES immediately. Please call NHDES to report a cyanobacteria bloom at (603) 848-8094 or email HAB@des.nh.gov.