Phytoplankton Monitoring Project Goal: To track the amounts and types of phytoplankton in Green Lake, as they are indicators of ecosystem health. This information allows us to understand and appreciate the base of the Green Lake food web and how it changes over time.

Green Lake phytoplankton photo gallery

What are phytoplankton? As you dip your toes in Green Lake, do you ever wonder about the microscopic organisms living there? Phytoplankton are free-floating microscopic organisms found in both freshwater and ocean, and they are big enough to see with an ordinary light microscope.  They are mostly algae, but also include cyanobacteria. They are mostly solitary cells, but some join into intricate colonies or filaments. They feed the world through photosynthesis, creating food from carbon dioxide, water and sunlight. 

Phytoplankton can rapidly multiply or die based on lake temperature, nutrient levels, sunlight, and amounts of predatory zooplankton.  Each week, water samples from two locations at the lake (North Dock, next to the west beach and Duck Island Launch, the small cove facing duck Island) are fixed in iodine solution, concentrated and then counted using a standard light microscope.  By regular counting, we can see their populations change in response to natural and human-caused environmental changes.

The most abundant types of Phytoplankton in Green lake, updated 5/6/26

What Cyanobacteria are currently in Green Lake? updated 5/26/26

Cyanobacteria qualify as phytoplankton because they do photosynthesis and float in the water, soaking up the sun. They are famous for blooming in nutrient-rich lakes like Green Lake, and they can sometimes produce dangerous toxins (cyanotoxins).  King County scientists test Green Lake for cyanotoxins each week during the summer months or any time there’s a bloom. You can find the toxin test results here: Washington State Toxic Algae. On 5/11/26, a cyanotoxin called microcystin was detected at the West Beach of Green Lake, so the beach was temporarily closed. The likely culprit was the small amount of Microcystis cyanobacteria showing up under the microscope lately. See below.

Successions of Phytoplankton

Because of constantly changing weather and nutrient conditions, the phytoplankton in many lakes never quite come to equilibrium and instead “take turns” thriving as their favorite temperatures and nutrient levels come and go.  This is a phenomenon in population biology called Succession. 

The graph below shows successions at North Dock.  This site gets plenty of circulation and is a pretty good representation of most of the lake.  As you can see, even in the deepest gloomiest part of our last Seattle winter, beautiful Asterionella diatoms (blue peak) were thriving.  When Asterionella died out, Cryptomonas took over.   What type (genera) will dominate next?  Will the same patterns repeat next year?  What is making this happen?  Stay tuned.

All photos and graphs by _{LKW, FOGL volunteer)}