Toxic Algae – updated 11/01/25

Cottage Lake Algae Watch:

Toxic Algae results (10/27/25) – Toxic algae warning lifted – samples tested below the threshold but could still be dangerous for pets. Details here.

Toxic Algae results (10/06/25) – Toxic algae warning – samples continue to test above the threshold value for microcystin so warning signs and fences will continue to stay up at the park. The lake is looking better after the rains, but please continue to be careful, avoid any visible algae, and keep your pets away from the water. Detailed results: https://www.nwtoxicalgae.org/Data.aspx?SiteID=23

Toxic Algae again detected in Cottage Lake (sample from 09/01/25): We received lab results for the toxic algae sample collected in Cottage Lake, and it indicated above-threshold values for microcystin. Please avoid any algae scums you see and consider the lake closed for recreation.

The toxic algae warning has been lifted (06/04/25) – We have received the results back from Monday’s sample and we now have two samples below State Recreational Guidelines – Public Health has lifted the lake advisory for toxic algae in Cottage Lake.

Toxic Algae again detected in Cottage Lake (sample from May 4th): We received lab results for the toxic algae sample collected on 5/4/2025 in Cottage Lake, and it indicated above-threshold values for Anatoxin-a. Please avoid any algae scums you see and consider the lake closed for recreation.
Detailed results: https://www.nwtoxicalgae.org/Data.aspx?SiteID=23

Toxic Algae again detected in Cottage Lake (sample from Oct 6th): We received lab results for the toxic algae sample collected on 10/6/2024 in Cottage Lake, and it indicated above-threshold values for microcystins. Please avoid any algae scums you see and consider the lake closed for recreation. Detailed results: https://www.nwtoxicalgae.org/Data.aspx?SiteID=23

Algae Update – 9/6/24: Good news! Toxin levels in Cottage Lake are below guidelines again this week (two weeks in a row) and the warning has been lifted. Public Health has let the park know, so the signs there should be coming down soon. As always, please avoid any algae scums you see and let me know where you see algae so we can sample if needed.

Toxic Algae detected in Cottage Lake (sample from Aug. 4th): We received lab results for the toxic algae sample collected on 8/4/2024 in Cottage Lake, and it indicated above-threshold values for microcystins and anatoxin. Please avoid any algae scums you see and consider the lake closed for recreation. Detailed results and history

2022: Algae blooms have been reported (mid-August) and some blue-green algae has been seen – so far, all tests have come back negative for toxins (see data on the NW Toxic Algae Site). Please continue to report any algae you see and, as always, avoid contact with algae scums. We have had very little rain this summer, so we will watch the lake carefully after the first heavy rain as the runoff may contribute to algae growth.

Treatments:

Phosphorus treatment done on 05/30/25 – we were able to work with Aquatechnex to squeeze Cottage Lake into their busy schedule on Friday, May 30th. This is the same treatment as last year – no restrictions on irrigation or on recreational use (although the lake is still officially under a warning for toxic algae) 
Aquatechnex put together a presentation showing that some progress was made with the treatments last year, but we still have to be patient before we see full results (I know it is disappointing to still have toxic algae warnings, but I do believe we are on the right path). The presentation has a comparison with another lake (Long Lake) that they are treating – it shows the progress they saw with multiple years of treatments.
Treatments are not cheap (we will limit this year’s treatment to $30K due to available funds). We had a couple of large $5K+ donations which allowed us to continue the treatment plan this year but are brainstorming fundraising ideas so we can continue next year. Let us know if you have ideas and thanks to everyone that continues to support Friends of Cottage Lake.

2024 Update – the lake was treated on May 31st and Aug 5th 2024 with lanthanum (eutroSORB G – https://eutrosorb.com/eutrosorb-g.html) 

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.

What Can You Do?
To minimize the amount of phosphorus your own activities may be contributing to nearby water, there are a number of very helpful things that people living in the watershed can do:

  • Minimize use of phosphorus cleaners. Clothes-washing detergents have been phosphorus free since the 1970s, but other cleaners often still contain it. Read the labels to see which products contain phosphorus and try the ones without it. With our soft water, they should be very effective.
  • Wash your car at a car wash that recycles its water, keeping soaps and road dirt out of the surface waters of the watershed. If you must wash your car at home, use water only.
  • Use low or no phosphorus fertilizers on your lawn. Grass needs nitrogen and potassium
    in fertilizers to become lush and green. It doesn’t need the phosphorus for good color or
    leaf production and the excess phosphorus will leach out when you water, getting into streams and lakes.
  • Don’t dispose of grass clippings in or near water.
  • Don’t feed the geese and ducks. They will congregate on beaches near good food sources and their feces will wash into the surface water and eventually into the lake.
  • Scoop your pet’s poop and dispose of it in the trash or into the septic system.
  • Keep your septic system maintained and functioning properly. Have it checked every three to five years.

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:

WA State Department of Health Information: https://www.doh.wa.gov/CommunityandEnvironment/Contaminants/BlueGreenAlgae

This chemical turns polluted green lakes clear. Is it safe?https://www.nationalgeographic.com/environment/2018/10/aluminum-sulfate-clears-polluted-lakes-algae-blooms/

Across U.S., Eruptions of Toxic Algae Plague Lakes, Threatening Drinking Water and Recreation
https://www.ewg.org/interactive-maps/2019_microcystin/

Useful Links:

Identification of blue-green algae:
https://www.nwtoxicalgae.org/Gallery.aspx
https://www.king5.com/article/news/is-that-algae-safe-how-to-identify-blooms-in-washington-waters/281-29e2f871-a167-4770-ae8c-ff35d97f8011

Possible Grant Sources:
https://ecology.wa.gov/About-us/How-we-operate/Grants-loans/Find-a-grant-or-loan/Freshwater-algae-program-grants#ApplicantGuidance

Environmental Working Group on Toxic Algae:
https://www.ewg.org/key-issues/water/toxicalgae

Testing History:

  • Aug 4th, 2024 – We received lab results for the algae sample collected on 8/4/2024 and it  indicated above-threshold values for microcystins and anatoxin (lake closed for recreation)
  • Sept 7th, 2021 – two weeks of samples below the limits, so the lake has been re-opened. There is still a lot of algae out there, so continue to be cautious.
  • Aug 23rd, 2021 – the sample submitted for toxin testing from Cottage Lake on 8/23/21 came back with a microcystin at level of 13 µg/L, exceeding the recreational use guideline of 8 µg/L. Based on this result, the lake is considered unsafe for swimming (see yellow warning here: https://www.nwtoxicalgae.org/HealthRisks.aspx)
  • Sept 27th, 2020 – sample tested positive for microcystin (positive again on Oct 11th, Oct 25th) – closed for the season (tested negative again in Dec 2020).
  • Aug 18th, 2019 – samples again “below the recreational guidance value” and therefore safe for water access
  • Aug 5th, 2019 – two weeks of good samples so the lake has been re-opened
  • July 21st, 2019 – lake closed and warning signs up (see https://www.nwtoxicalgae.org/ for latest status)
  • August 30th, 2018 – lake closed and warning signs up (lasted until early October)
  • Sept 2017: King County ​tested the sample we gave them and it contained high levels of microcystins, which is a common toxin produced by blue-green algae. The lake was closed (no swimming, keep pets out) for several weeks (until the samples were below the health limit). This was the third year in a row where we had a toxic algae bloom at the end of a hot summer.