Toxic Algae

Cottage Lake Algae Watch:

The sample from 8/18/19 was “below the recreational guidance value” and therefore safe for water access. Please continue to be cautious if you see a thick algae bloom and continue to share your pictures and questions.

Latest Toxic Algae detected in a sample from July 21st, 2019 – see

Lake Water Quality Issues:

Cottage Lake contains high phosphorus concentrations and is therefore on Washington State’s 303(d) list of lakes not meeting state water quality standards. High phosphorus levels lead to increased algae blooms that impact water quality and the long-term usability of a lake. These algae blooms, when severe, are potentially toxic to humans. Past studies show that phosphorus is coming primarily from incoming streams (Daniels Creek and Cottage Creek). Other suspected sources include harmful lawn-care products and ill-maintained septic systems.

Algae Blooms and Cyanobacteria (blue-green algae):

Due to the high phosphorus levels in the lake, Cottage Lake is home to a lot of algae (microscopic single-celled plants). During hot, sunny weather, the algae multiplies quickly and this is referred to as an “algae bloom”(algae forms large dense patches visible on the surface of the water). Most species of algae are harmless and the blooms are just a nuisance. Cyanobacteria (commonly referred to as “blue-green algae”, but really a bacteria) is another organism living in the lake that multiplies during warm, sunny weather – again, cyanobacteria is generally harmless, but some can produce toxins which can be harmful to animals and humans. In particular, Microcystis (a type of cyanobacteria) has produced toxin blooms in other lakes in our area in the past (including Lake Sammamish). Another type of cyanobacteria has also been linked to swimmer’s itch.

Articles on Algae or treatments:

This chemical turns polluted green lakes clear. Is it safe?

Across U.S., Eruptions of Toxic Algae Plague Lakes, Threatening Drinking Water and Recreation

Useful Links:

Identification of blue-green algae:

Possible Grant Sources:

Environmental Working Group on Toxic Algae: