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New Hampshire Department of Environmental Services

Media Center

DATE: October 7, 2020
CONTACT: Amanda McQuaid (603) 271-0698 (O), 848-8094 (C)

State Changes Cyanobacteria Advisory to Alert Status for Spofford Lake in Chesterfield, New Hampshire

Concord, NH - Cyanobacteria have been observed on Spofford Lake, appearing as black accumulations along some of the shorelines. The cyanobacteria have come from the bottom of the lake and are referred to as benthic mats. These mats can sometimes dislodge from the lake bottom, rise to the surface, and are carried by winds and currents to the shorelines.

The advisory was issued on September 10, and since then the shorelines accumulations have diminished. Though the cyanobacteria are still present and we are unaware of the extent of the mats at the bottom of Spofford Lake. Since there is not an active bloom in the lake water, NHDES has changed this advisory to an alert. Cyanobacteria cell concentrations in the lake water are minimal. However, the build-up of these mats on the shore can present a potentially toxic accumulation of cyanobacteria. Please avoid contact with the black mats and keep pets and children away.

Cyanobacteria 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. 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 September 10, 2020 and was changed to an alert on October 7, 2020 as shorelines accumulations continue to diminish. The mats are still present and NHDES advises that lake-goers avoid contact with these mats of cyanobacteria.

Spofford Lake in Chesterfield NHSpofford Lake in Chesterfield NH
Shoreline accumulations of cyanobacteria on September 10 and October 7, 2020 along Spofford Lake

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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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

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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