DEP has confirmed the presence of Cyanobacteria on Damariscotta Lake in the South Arm. It has also been identified in Damariscotta Mills (see photo below).

Linda Bacon of Maine DEP has stated that the current volume cyanobacteria that she has observed on Damariscotta Lake does not appear to be enough to create toxic conditions. However, precautions should be taken to stay out of areas with heavy accumulations, such as the scum that forms on the shoreline. Do not allow children to play in those areas, do not ingest this water nor allow your pets to drink this water, and shower after swimming. 

If you live on the South Arm and have a drinking water intake, purchase bottled water for drinking (boiling and reverse osmosis do not remove cyano-toxins) and take brief showers (cyano-toxins can become airborne). Additional information can be found on the state’s webpage:

The record hot weather we have had this summer has raised the water temperature substantially. That, combined with ongoing inputs of nutrients, have created this unsettling issue.

Our Invasive Plant Patrol volunteers and Midcoast Conservancy staff are actively patrolling for this threat.  We will continue to keep lake users informed as we work with the Maine DEP to monitor conditions.

A map of lake testing sites can be found here.

UPDATE: On Saturday, August 15th, Midcoast Conservancy staff and volunteers, along with Peter Countway of Bigelow Laboratory, were on the lake to collect water samples for analysis. Results of the testing show that toxicity levels of the cyanobacteria in the lake are insignificant at this time, based on EPA standards.

Swimming in the lake is safe, although people are encouraged to avoid areas where yellow or blue-green scum occurs—particularly with children and dogs. Drinking water sourced from the lake is acceptable but using bottled water when possible is suggested.

For assistance with finding solutions to any of the above issues, please contact Midcoast Conservancy. For any questions, or if you spot this in Muscongus or Great Bay, please email Patricia at or Linda Bacon at

When reporting any potential issues, please include where you saw the cyanobacteria and if the cyanobacteria present in low, medium or high density (see photos below for examples).



What is cyanobacteria?

Cyanobacteria (often called blue-green algae) are aquatic bacteria that photosynthesize and have been around for over 3.5 billion years. When conditions are right their numbers can explode causing what we call a “bloom”. Cyanobacteria blooms can present as paint-like smears in the lake or simply turn the water green. Additionally, cyanobacteria are able to regulate their position in the water column so the density can appear to change throughout the day and be different day to day. 

Is it toxic?

On August 10th, Linda Bacon, with the Maine Department of Environmental Protection identified the type of cyanobacteria as Oscillatoria sp., which is capable of producing toxins that are neurotoxic to humans, pets and wildlife. Linda observed the cyanobacteria in the lake and stated that the current quantities found in the lake were unlikely to be toxic. Degree of exposure is important, the more contact with the cyanobacteria the more potential for toxicity. This can be because of high density of cyanobacteria or prolonged exposure. Avoid areas of high concentration such as scum lines along the shoreline in windy conditions. Do not allow children to play in those areas, do not ingest this water nor allow your pets to drink this water, and shower after swimming. If you live in the South Arm of the Lake you should not use your lake intake for drinking water as reverse osmosis and boiling does not remove cyanotoxins. Use bottled water for drinking and take quick showers as cyanotoxins can become aerosolized and cause health problems. .

What caused this problem?

The record high air and water temperatures combined with excess nutrients (primarily phosphorus) create ideal conditions for algae and cyanobacteria growth. There are multiple ways phosphorus enters a lake, including erosion, fertilizers, septic systems, agricultural sources, and others.

Will it spread to the other basins?

It will likely will not spread from one bay to another, but water chemistry, and nutrient loading in the other bays may lead to presence of nuisance algae and cyanobacteria. 

Signs and symptoms of exposure: nausea, vomiting, diarrhea.

What can I do? 

What is Midcoast Conservancy Doing?

  • We are  working with DEP to track the status of the cyanobacteria
  • We have been collecting water quality data, including phosphorus samples in Great Bay, Muscongous Bay, and South Arm since 1977. We track nutrients levels at the surface, mid depth and at the bottom of the lake to track changes over time 
  • We tackle large scale erosion projects on gravel roads, driveways and shoreline, see the Damariscotta Lake Protection Plan
  • Donate to the $15,000 Damariscotta Lake Challenge Gift. A generous donor will match dollar for dollar every new Midcoast Conservancy membership within the Damariscotta Lake shoreline and surrounding area.