What are cyanobacteria?

Cyanobacteria are some of the earliest inhabitants of our waters. They naturally occur in our lakes, usually in relatively small amounts. However, when nutrient pollution increases in our lakes, the amount of cyanobacteria increases in our lakes.

New Hampshire’s lakes have seen a record number of toxic cyanobacteria blooms for the past four summers. These blooms can produce toxins that make people, pets, and wildlife sick.

What to do if you see or suspect a cyanobacteria bloom:

  • Don’t wade or swim or drink the water.
  • Keep kids, pets, and livestock out of the water.
  • Anyone (including pets) who comes in contact with a bloom or scum should rinse off with fresh water as soon as possible.

Report it!

Cyanobacteria Information Sheet

We worked with the New Hampshire Department of Environmental Services to create a cyanobacteria identification sheet that includes information on what to do if you see a bloom, how to report it, and how to stay informed about advisories.

Image of the various ways Cyanobacteria blooms may appear.

What does a bloom look like?

When present in low numbers, cyanobacteria can’t be seen with the naked eye and typically do not cause problems. But, when the perfect storm of “bad conditions” come together—such as heavy rains followed by a few days of sunny, warm weather— cyanobacteria blooms may occur in our  lakes. 

When blooms are discovered, they are usually found in cove areas or along the windward shoreline of a waterbody. The water’s surface may look like pea soup, antifreeze, or someone dumping greenish-blue paint into the water. Some blooms can look  cloudy white or like little yellow fuzzy balls. Alarmingly, we’re seeing and learning about more and more types of cyanobacteria each year. 

Blooms MUST be tested to determine if they contain toxins or not. It isn’t possible to tell if a bloom is toxic just by its appearance.

What you can do:

Image of the various ways Cyanobacteria blooms may appear.

Report Cyanobacteria Blooms

Use the New Hampshire Department of Environmental Services Cyanobacteria Reporting Form. Include photos, lake name, and the general location of the bloom.

Help Prevent Cyanobacteria Blooms - Participate in the LakeSmart Program

Learn how to reduce the amount of nutrients you contribute to cyanobacteria blooms. LakeSmart is a free, non-regulatory, and voluntary education and award program for property owners to maintain their homes and property in ways that protect lake water quality, wildlife habitat, and property values.


Kathy Bridgeman Award for LakeSmart
Image of Advisory Sign for Cyanobacteria with Bloom in the Background

Sign-Up for Cyanobacteria Alerts

The New Hampshire Department of Environmental Services posts public health advisories when fecal bacteria or cyanobacteria counts at public beaches exceed the state standards.

Visit their website to sign up for weekly updates, get waterbody-specific notifications, and view the healthy swimming mapper.