WINCHESTER, N.H. (MyKeeneNow) — A small but potentially dangerous cyanobacteria bloom has been detected along the northern shoreline of Forest Lake, prompting state officials to urge swimmers and boaters to steer clear of the affected area.

The New Hampshire Department of Environmental Services confirmed the presence of the bloom after it was first reported on Aug. 6. From the observer’s vantage point, between half and three-quarters of the lake’s surface was visible, but the bloom itself was confined to less than 5 percent of the waterbody. The growth appeared as fine specks suspended in the water column.

Laboratory analysis of a sample taken the following day found concentrations of Anabaena too numerous to count — a level that can produce harmful cyanotoxins. Officials warn that blooms can shift quickly with wind, waves, or boat traffic, meaning other parts of the lake could also become affected without warning.

Cyanobacteria, also known as blue-green algae, occur naturally in freshwater but can multiply rapidly under favorable conditions. Contact with the water can cause health problems for humans and animals, ranging from mild skin irritation to serious illness. Children and pets are considered especially at risk.

Unlike bacterial contamination advisories, which can trigger temporary beach closures, cyanobacteria warnings do not result in the state closing a lake. Instead, officials recommend avoiding areas where blooms are visible and staying alert for changes in water color or texture.

The U.S. Environmental Protection Agency notes that climate change may be contributing to more frequent and intense blooms across the country.

Residents can track advisories, including the current Forest Lake warning, through the state’s Healthy Swimming Mapper at www.des.nh.gov/water/healthy-swimming/healthy-swimming-mapper.