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New Hampshire Department of Environmental Services

Media Center

DATE: June 29, 2020
CONTACT: Amanda McQuaid (603) 271-0698 (O), 848-8094 (C)

State issues Cyanobacteria Advisory for Silver Lake in Hollis, NH

Concord, NH - A cyanobacteria bloom has been observed on Silver Lake in Hollis, NH. An advisory was issued on June 9 and was removed on June 19. There is now another wave of the bloom accumulating along shorelines. The bloom was reported on June 29 and the New Hampshire Department of Environmental Services (NHDES) will follow-up with sample collection for details. Previous samples contained Microcystis and Woronichinia (cyanobacteria). Advisories are issued when cyanobacterial cell concentrations exceed 70,000 cells/ml and alerts are shared when blooms are visible. In this event, NHDES can confirm that areas where blooms are visible have elevated concentrations of cyanobacteria. As a result, NHDES has issued a second cyanobacteria advisory for this lake (this summer) 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.

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 June 29, 2020 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
Updates on advisories and warnings may be obtained at
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NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
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