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New Hampshire Department of Environmental Services

Media Center

DATE: June 8, 2020
CONTACT: Amanda McQuaid (603) 271-0698 (O), 848-8094 (C)

State Issues Cyanobacteria Advisory for Silver Lake in Hollis, New Hampshire

Concord, NH - A cyanobacteria bloom has been observed on Silver Lake in Hollis, NH. Samples were collected on June 8, 2020 at approximately 10 am. The accumulation appeared as thick aggregates of pollen. However, the samples also contained Microcystis and Woronichinia (cyanobacteria). Cell concentrations were 327,500 cells per milliliter of lake water at the beach. Advisories are issued when cyanobacterial cell concentrations exceed 70,000 cells/ml. As a result, the New Hampshire Department of Environmental Services (NHDES) has issued a cyanobacteria 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.

The bloom does not show blue-green colors as it is mixed in with thick yellow pine pollen and another organism called Botryococcus, which makes it also look orange. 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 9, 2020 and will remain in effect until NHDES confirms that cell concentrations of the bloom have subsided.

A cyanobacteria bloom has been observed on Silver Lake in Hollis, NH. The bloom is mixed in with pollen and is hard to distinguish, yet cell counts were 327,500 cells/ml from samples collected on June 8, 2020. This photo was taken exactly one year ago and conditions currently appear identical to this.

cyanobacteria bloom on Silver Lake in Hollis, NH

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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