When it comes to cyanobacteria, only you can decide the level of risk you are comfortable with. Always perform a self-assessment before recreating in the water. When in doubt, err on the side of caution to avoid potential exposure. Below are some techniques and resources for performing a risk assessment to help you make a decision that is right for you.
Before You Enter the Water
Before swimming, boating, or letting pets in the water, take a close look at conditions. Check for changes in color, clarity, or texture on the surface. Cyanobacteria can appear in many forms: scattered dots, streaks, clouds, or thick, paint-like mats. For example, Gloeotrichia appears as tiny yellow-green specks just below the surface. This species has been widely reported around Lake Wentworth and Crescent Lake over the past week.
When you spot cyanobacteria, consider how dense it appears:
- Light, scattered colonies (like a few stars in the night sky) generally indicate lower risk.
- Dense clusters (like a galaxy, with thick clouds or surface mats) are a sign to stay out of the water and keep pets out until conditions improve.
Blooms can shift quickly with wind and weather, so conditions may vary across different shorelines or even within a single day. One area may look clear while another is dense. Only swim when you have self-evaluated the risk in that area.
Who Monitors Cyanobacteria?
NHDES is responsible for collecting and analyzing samples to confirm blooms across all 1,000 lakes in New Hampshire. As the State agency, only NHDES can issue official Alerts and Warnings. WWA partners with NHDES to help get the word out, helping the public make informed decisions.
NHDES has limited staff resources, with only one full-time employee evaluating samples and updating the Healthy Swimmer Mapper, supported by three interns who collect samples statewide. Real-time updates are not possible, so patience is essential. If cyanobacteria are present, only you can decide what level of risk is acceptable for yourself.
This video from NHDES provides additional guidance on performing your own risk assessment:
Understanding the Healthy Swimming Mapper
The NHDES Healthy Swimming Mapper is updated on weekdays during the cyanobacteria bloom season. It shows the status of cyanobacteria observations, watches, and warnings across New Hampshire.
This week, Lake Wentworth and Crescent Lake show many black triangles because WWA’s engaged membership is submitting “bloom watch” reports when cyanobacteria are observed. Thank you for helping track conditions!
*The mapper is updated on weekdays during the months when cyanobacteria blooms are most likely to occur.
How Reports Are Displayed on the Mapper:
- Black Triangle: A photo report was reviewed, and it is likely a cyanobacteria bloom, but no sample has been collected
- Orange Triangle: A sample was collected, and the cyanobacteria cell density was approaching but did not exceed 70,000 cells/mL
- Red Triangle: A sample was collected, and the cyanobacteria cell density exceeded 70,000 cells/mL
You can click on each triangle to see the date of the observation, pictures from that report, and any counts or species information available.
Warnings vs. Watches
- Warning: Issued when multiple samples exceed 70,000 cells/mL.
- Watch: Issued based on a single sample exceeding the recreational threshold, a photo report, or when bloom material has passed.
Remember: reports reflect a static point in time and may not represent current conditions. Cyanobacteria blooms are highly dynamic. A visual assessment is always recommended before recreating or allowing pets in the water. NHDES advises avoiding contact in areas where bloom material is observed.
For more help, see the WWA’s Guide to Navigating the Healthy Swimming Mapper.