Algae Blooms FAQs

Click here to be notified of the latest updates and notifications on ponds and other Public Health topics.

What steps does the Town take when a resident reports a bloom?
What water bodies are tested in Plymouth?
  • Only public bathing beaches are tested for e-coli bacteria per state health regulations, due to the large number of water bodies and expense of testing.
  • Ponds with active blooms are identified and confirmed by visual observation for cyanobacteria by the state. 
  • Associations without public access are encouraged to post this sign in visible areas once a bloom is confirmed.
    • It is up to private pond associations to incorporate testing into their management plan, should they wish to do so.
  • Advisories are not lifted until the water has been consistently clear for TWO WEEKS
How do I report a cyanobacteria bloom?
What are the health concerns associated with cyanobacteria?

A confirmation of cyanobacteria bloom does not necessarily indicate it is toxic.  Cyanobacteria MAY give off microcystin toxins.  However, all cyanobacteria blooms are treated as potentially toxic to ensure public health.

  • Skin irritation, stomach cramps, vomiting, nausea, diarrhea, fever, sore throat, headache, muscle and joint pain, blisters of the mouth and liver damage.
  • Swimmers in water containing cyanobacterial toxins may suffer allergic reactions, such as asthma, eye irritation, rashes, and blisters around the mouth and nose.
  • Animals, birds, and fish can also be affected by high levels of toxin-producing cyanobacteria.
    • If your animal goes into the water, be sure to rinse them off thoroughly
    • If your animal ingests water and shows signs of illness, contact your veterinarian
  • Advisories are not lifted until the water has been consistently clear for TWO WEEKS
What does cyanobacteria look like?

Bright green film that looks like “paint” in the water.  May contain small white specs in the water column. Looks like an oil slick, green specks, pea green layer on surface (Click here and here for pictures) or visit for more examples. 

  • Floating algae
  • It has a fuzzy appearance and is often green in color with blue highlights.
  • Has a musty odor
How do I know if a pond has been identified as having an active cyanobacteria bloom?
What are the causes of cyanobacteria?
  • Cyanobacteria are naturally occurring
  • Waterbodies with increased nutrients (phosphorus or nitrogen) will potentially feed the cyanobacteria into a bloom. 
  • Sediments in the bottom of ponds contain phosphorus from continuous loading from agriculture, septic systems, stormwater, rainfall, fertilizers.  Under low oxygen environments when temperatures increase can decrease the level of oxygen in the water column thereby releasing the phosphorus from the sediment which feeds the cyanobacteria. 
  • Increased rainfall
  • Hot temperatures
  • Changes in Ph (phosphorous and/or nitrogen)
  • Failing cesspools/septic systems.  Even non-failing systems contribute nutrients as septic systems only treat for bacteria not phosphorus or nitrogen. 
Where can I learn more about Cyanobacteria?