Mirror Lake

Tuftonboro, Wolfeboro, NH


Type of Notification Date Issued Date Removed # of Days Issued Dominant Taxa Initial Cyanobacteria Cell Density (cells/mL)
Warning 8/12/2008 12/1/2008 111 Oscillatoria >70,000 or >50%
Warning 9/27/2014 9/30/2014 3 unidentified >70,000
Warning 8/7/2019 8/20/2019 13 Microcystis 1,329,500
Warning 8/12/2021 8/20/2021 8 Dolichospermum 115,000


If you observe a cyanobacteria bloom, please submit a Bloom Report Form.


Current Warnings are listed on the Healthy Swimming Mapper.


The Cyanobacteria Harmful Algal Bloom Program monitors waterbodies when a potential cyanobacteria bloom is reported by a member of the public. A cyanobacteria Warning (Advisory) is issued lake-wide when cyanobacteria cell concentrations exceed the recreational threshold of 70,000 cells/mL. Warnings are not based on toxin evaluation but are intended as a precautionary measure for when short-term exposure to cyanotoxins is likely. When a Warning is issued, resampling is performed weekly until the bloom subsides. Warnings are issued from May 15 through October 15.

A cyanobacteria Alert is issued based on a photo before NHDES can analyze a sample; if the cyanobacteria density is approaching the recreational threshold but has not yet exceeded it; or if the bloom has been reported to have passed but may reoccur. Resampling only occurs if further bloom reports are submitted. Alerts are issued year-round as needed.

Cyanobacteria blooms are potentially toxic to domestic animals, livestock, wildlife, and humans due to the cyanotoxins produced. The type and amount of toxin produced varies over time and from lake to lake. Cyanotoxins can cause both acute and chronic illnesses that can target the skin, liver, kidney, and central nervous system. Acute effects, such as skin and mucous membrane irritations, can occur after short-term exposure with water containing cyanotoxins. Chronic effects, such as liver, kidney, and central nervous system damage, can occur over a long period of time from ingesting water containing toxins. For more information on cyanobacteria and the program, see our Frequently Asked Questions. \[\\[0.03in]\]

Follow these steps to minimize immediate risks:

  • Always perform a visual risk assessment looking for discoloration or unusual growth.
    • Report suspected blooms not already on the Healthy Swimming Mapper through the Bloom Report Form.
  • If a Warning is posted or the water looks suspicious:
    • Do not wade, swim or drink the water.
    • Keep pets and livestock out of the water.
    • Wash with fresh water and soap if you’ve come into contact with suspected cyanobacteria.
  • When in doubt, stay out!