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FAQs

How to spot harmful algal blooms and what to do when you see them.

What are cyanobacteria?  

A collage of examples of cyanobacteriaCyanobacteria (formerly called blue-green algae) are photosynthetic bacterial microorganisms that are ubiquitous throughout the world. Cyanobacteria are naturally occurring in lakes and ponds and appear at high concentrations in the form of a bloom when there are excess nutrients in the water, often due to runoff and stormwater washing in fertilizer and other contaminants. A bloom can appear as a blue-green scum at the water’s surface or turn an entire waterbody green. However, there are several color variations of blooms and various habitats where cyanobacteria thrive. Due to the nature of cyanobacteria, it can be difficult to track a bloom. This is because it may be visible in one area in the morning, but in a few hours it can appear to have vanished or dispersed by wind, current and/or rain. 

What do cyanobacterial blooms look like?

A cyanobacterial bloom can appear as a blue-green scum at the water’s surface or turn an entire waterbody green. However, there are several color variations of blooms and various habitats where cyanobacteria thrive. Due to the nature of cyanobacteria, it can be difficult to track a bloom. This is because it may be visible in one area in the morning, but in a few hours it can appear to have vanished or dispersed by wind, currents and/or rain. 

Can I swim at a beach if cyanobacteria are present? 

Once a bloom or surface scum forms, cyanobacteria can present a potential health risk to humans and pets due to potentially high concentrations and its toxicity. The NHDES recommendation is to avoid contact with any area of water experiencing a visible bloom and report your findings to NHDES. Pets and children especially should NOT make contact with the water as ingestion of the bloom/scum is a primary concern.  

When is an advisory issued for cyanobacteria? 

Samples are collected and microscopically analyzed at the NHDES Jody Connor Limnology Center (JCLC). An advisory will be issued if cell concentrations of cyanobacteria exceed state standards. Any advisories that are issued are followed up with additional inspections until the cell concentrations are below the state standard. The microscopic evaluation is completed to determine the taxa and concentrations in order to inform and protect public health.  

What are the New Hampshire state standards for cyanobacteria?

The standard for cyanobacteria is either cyanobacteria making up 50% of the total cell count or having a cyanobacteria cell count of greater than 70,000 total cells per ml of water. Beach advisories are issued when samples taken from scum at a beach are either more than half cyanobacteria cells or the cell concentration is greater than 70,000 cells per ml of water.

What should I do if I suspect a cyanobacteria bloom? 

If you suspect a cyanobacteria bloom, make sure that children and pets do not drink, wade, swim, or make contact with the water. Call the Harmful Algal and Cyanobacterial Bloom Hotline to report the bloom (603-848-8094). If you are able, take a picture of the bloom and send it to the hotline to assist in reporting the bloom to a limnologist. A bloom can last anywhere from a few hours, to days or months. Pay attention to the changing conditions of your individual shoreline for the possibility of a bloom. 

How can a cyanobacteria bloom be removed or controlled? 

Cyanobacteria are naturally occurring and cannot be eradicated. However, there are ways to prevent their excess growth. To decrease the likelihood of a cyanobacteria bloom, continued watershed management practices that reduce nutrient loading to the water must be implemented. Be aware of the Shoreland Protection Act and its restrictions on the use of fertilizers along the shoreline. No fertilizer, except limestone, may be used within 25 feet of the reference line (shoreline). Pick up after your pets and be aware of the potential for septic systems to fail. Managing blooms can be lake-specific, and careful consideration of the lake uses, ecology and watershed must be considered.