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New Hampshire Department of Environmental Services

Media Center

DATE: October 5, 2018
CONTACT: Amanda McQuaid (603) 271-0698 (O), 848-8094 (C)

State REMOVES Cyanobacteria Lake Warning for Marsh Pond in New Durham, NH

Concord, NH – The New Hampshire Department of Environmental Services (NHDES) has REMOVED a cyanobacteria lake warning issued on 7/27/2018 for Marsh Pond in New Durham. NHDES issues lake warnings for cyanobacteria when concentrations exceed 70,000 cells per milliliter of lake water. This waterbody has been carefully monitored by NHDES, UNH (University of New Hampshire) and local volunteers. Marsh Pond, Jones Pond and Downing Pond (New Durham) were collectively examined for research purposes through the University of New Hampshire. Local volunteers were able to observe and report daily cycles of the blooms surfacing throughout Marsh Pond and Jones Pond, while no cyanobacterial blooms were observed on Downing Pond. Samples have been collected nearly every week to observe the conditions of the waterbodies since early May. Volunteers collected samples for microscopic analyses. Duplicate sampling efforts and weekly communication were conducted between NHDES, UNH and volunteers. A deep layer bloom in Marsh Pond had been evident for the duration of the advisory. The bloom was primarily comprised of Oscillatoria, estimated to be 180,000 cells/ml from a deep layer in the lake on July 20. This deep layer bloom increased to over 19 million cells/ml on July 27. Samples were collected from the surface water on July 27, revealing estimates of 140,000 cells/ml of Oscillatoria at the surface. Globs were often observed near shorelines in the morning, containing visibly high concentrations of cyanobacteria throughout the lake. Marsh Pond was sampled again August 3, where it was evident that the bloom was still highly concentrated ranging from 80,000 to 120,000 cells/ ml from surface waters. On August 9, the bloom was observed deeper in the lake (~ 50 million cells/ml at ~ 4 meters). Samples were collected again on August 16 when large aggregates were surfacing. Samples were collected between 0-3 meters of lake water and cell concentrations varied, though remained elevated at roughly 187 million cells/ml from the surface waters. On August 24, the deep layer bloom was estimated to be 22 million cells/ml from a 5 meter depth sample. Surface aggregates contained over 300 million cells/ml. On August 30, surface grab samples contained over 19 million cells/ml, while the count for the deep layer bloom (at 5 meters) was nearly 42 million cells/ml. Additionally, an integrated sample from 0-3 meters of water was estimated to contain 2.655 million cells/ml. Occasional sightings of surface accumulations continued into September, but appeared to be an early morning event. By September 6, the bloom had mostly dissipated from the surface waters; however the deep layer bloom was still present until the end of September. The monitoring effort by UNH and volunteers continues, with the most recent samples collected on October 4. There were no observations of cyanobacteria in Marsh Pond on October 4, 2018. Please continue to monitor your individual shorelines for changing conditions.

Cyanobacteria are natural components of water bodies worldwide, but blooms and surface scums may form when excess nutrients are available to the water. Some cyanobacteria produce toxins that are stored within the cells that can be 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 include liver and central nervous system damage.

NHDES advises lake users to avoid contact with the water in areas experiencing elevated cyanobacteria cell conditions typically where lake water has a surface scum, green streaks or blue-green flecks aggregating along the shore. NHDES also advises pet owners to keep their pets out of any waters that have a cyanobacteria bloom.

NHDES routinely monitors public beaches and public waters of the state for cyanobacteria. Once a cyanobacteria lake warning or beach advisory has been issued, NHDES returns to affected waterbodies on a weekly basis until the cyanobacteria standards are again met.

The warning went into effect on July 27, 2018, and was removed on October 5, 2018.

Visit the NHDES Beach Program website for photos and more information about cyanobacteria at
Updates on advisories and warnings may be obtained at
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.


A cyanobacteria bloom on Country Pond was observed on September 26, 2018. There have been no reported sightings since.

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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