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.

Some of the Phytoplankton in Green Lake _{(photos by LKW, FOGL volunteer)}

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.  By regular counting under the microscope, we can see their populations change in response to natural and human-caused environmental changes.

Cyanobacteria (Blue green algae) 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. Fortunately, our cyanobacteria of interest are large enough to see easily in a standard microscope.

What is the microscope telling us about Green Lake recently?

Cyanobacteria bloom

You might have noticed a greenish scum in the water along the shores of Green Lake recently, getting moved around by the wind. What is it? As shown above, the microscope reveals Dolichospermum cyanobacteria. Microcystis has also been showing up recently in some of the shore scum.  One or all of these cyanobacteria are likely making the microcystin toxin currently detected in Green Lake. It’s a good idea to stay out of the water for now!

Below are the currently most abundant types of phytoplankton in Green Lake*. Last updated 9/22/25

* How the monitoring is done: 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 Lugol’s iodine solution, concentrated and then counted by a volunteer scientist with a microscope. 

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.

Graph last updated 8/5/25