Cyanobacteria

At a Glance

Peak Bloom Season

June – Sept

Warm, calm, sunny days are the highest-risk window

Bloom Formation

24–48 hrs

Ideal conditions can produce a visible bloom rapidly

Primary Driver

Phosphorus

Excess nutrients feed bloom growth

What are Cyanobacteria?

Often called 'blue-green algae,' cyanobacteria are bacteria that can multiply rapidly under the right conditions and form harmful algal blooms (HABs) that are toxic to humans, pets, and wildlife.

Not True Algae

Cyanobacteria are prokaryotic bacteria, not algae. They photosynthesize like plants but fix atmospheric nitrogen, making them hard to control once established.

Thrive in Warm, Calm Water

Surface scums can appear overnight when phosphorus, warmth, and light align.

May Produce Toxins

Some strains produce cyanotoxins that harm humans, pets, and wildlife. You cannot tell if a bloom is toxic by sight — treat all blooms as potentially hazardous.

Be on the Lookout for Cyanobacteria Blooms (PDF)

Appearance

Look for pea-soup green, blue-green, brown, or reddish discoloration — like spilled paint, floating mats, or streaks along shorelines. Blooms often concentrate downwind and near shore, especially on calm mornings.

Smell

Musty, earthy, or grassy odors — similar to rotting vegetation — are a warning sign.

When in Doubt

Treat it as a bloom and stay out. Toxins can persist even after water looks clear.

Health Risks

Exposure can cause effects from mild irritation to severe illness, depending on toxin type and level.

Human Health

Skin contact can cause rashes, hives, and irritation. Swallowing water can cause nausea, vomiting, diarrhea — and in severe cases, liver or neurological damage.

Pet Safety

Pets are especially vulnerable — toxin ingestion can be fatal within hours. Keep them out of discolored water and away from shoreline scum.

Rising Phosphorus Levels

NH DES VLAP monitoring shows an increase in phosphorus since 2022, when we first experienced cyanobacteria blooms. Small increases can tip the balance. See what can be done to reduce phosphorus in the watershed.

Motor Boats & Sediment Disturbance

Motor boats, especially wake boats, churn up phosphorus-rich waters from deeper in the lake and from shoreline and bottom sediments, releasing nutrients into the warm surface waters, thus creating conditions that promote cyanobacteria blooms.

Wake boats are a particular concern because they can mix water down to 30 feet or deeper. Nutrient concentrations in deeper waters throughout the lake are often higher than at the surface. When boats churn up this nutrient-rich water into the warmer, light-rich surface layer, it creates exactly the conditions that promote cyanobacteria blooms. This is why Pleasant Lake asks that ALL motor boats operate in at least 30 feet of water, which means staying out towards the middle of the lake.

Warm Summers & Extended Calm Periods

Warmer summers extend the bloom-favorable window. Early observation and rapid reporting protect the lake and alert other users.

What to Do

Take these steps if you see a suspicious bloom.

Stay Out of the Water

Avoid swimming, wading, or water sports in or near discolored water. Keep children away from shoreline scum.

Keep Pets Away

Keep pets on a leash. Do not let them drink or wade near discolored water or shoreline scum. Rinse immediately if they had contact.

Report the Bloom

Report suspected blooms using the NH DES Cyanobacteria Bloom Report Form. Include photos, location description, and GPS coordinates if possible.

Stay Informed

Check the NH DES Healthy Swimming Mapper for the current bloom status in the lake, but realize that the state has no regular sampling for cyanobacteria in Pleasant Lake.

Watch: CyanoHAB Talk

Watch: CyanoHAB talk from a resident of a nearby lake who has studied cyanobacteria extensively.

PLPA volunteers monitor water conditions throughout the season and share what they are learning with the community.

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