State Issues Cyanobacteria Warnings on Lake Winnipesaukee

CONCORD, NH – Cyanobacteria blooms have been reported on Lake Winnipesaukee and two warnings were issued on June 12, 2024. The specific affected areas are Carry Beach and Brewster Beach in Wolfeboro, and 19-Mile, and Tuftonboro Neck in Tuftonboro.   Any surface scum, no matter the color, should be avoided to prevent toxin exposure. Do not recreate, let children play in, or let pets in any surface blooms. NHDES will resample the affected areas on June 19 and will continue weekly resampling if the bloom continues.

The cyanobacteria blooms are occurring as green clouds of material accumulating along shorelines. The cyanobacteria identified is Dolichospermum, in densities up to 561,000 cells/mL. The bloom material is mixed with dense pollen to varying degrees. At times, the cyanobacteria material is distinct from the pollen, appearing as green clouds separate from the yellow pollen clouds. In other areas the pollen and cyanobacteria are more mixed, appearing just as yellow clouds. See below for varying bloom presentation. Microscopic review indicates the pollen is masking the presence of cyanobacteria.

Cyanobacteria blooms with pollen mixed in. Left: 19-Mile Bay, Right: Tuftonboro Neck.

Where the bloom has been reported and where samples were collected are indicated on the maps below. Red points indicate severe bloom presentation and/or cyanobacteria recreational threshold exceedance (>70,000 cells/mL). Yellow points indicate additional test sites below the recreational threshold.

Warnings are issued when the cyanobacteria cell density exceeds the recreational health threshold of 70,000 cyanobacteria cells/mL. This density is associated with the level at which toxins may be present at a concentration that threatens public health. Exposure primarily occurs through ingestion and skin contact. Symptoms of exposure to cyanobacteria vary but can include skin irritation, stomach cramps, vomiting, nausea, diarrhea, fever, sore throat, headache, muscle and joint pain, mouth blisters, and acute liver damage. When there is a warning, NHDES advises lake users to avoid contact with the water. Pets and livestock should also be kept out of the water. 

NHDES recommends staying out of the water when a cyanobacteria warning is posted. This includes swimming, wading and any activity that may result in direct contact with the water. NHDES also recommends against using the water for drinking even when boiled or filtered, dish washing, bathing and other household uses when a warning is in place. Children and pets are particularly vulnerable to cyanotoxins.

Cyanobacteria blooms are dynamic and can change hour to hour based on wind, wave and boat action. Please see our FAQs for more information. Always perform a self risk assessment prior to recreating in any freshwater. This includes looking at the water for any discoloration or unusual growth and checking our resources. The Healthy Swimming Mapper has the latest sampling and warning information, and you can sign up for sampling result emails through the waterbody specific sign up form. If you see additional areas experiencing blooms, please take pictures and report it to us through the bloom report form. When in doubt, stay out of water with suspected blooms.

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