Phytoplankton are free-floating microscopic organisms found in both freshwater and ocean. They feed the world, because through photosynthesis they can create food from CO2, water and light from the sun. They are usually composed of single cells, some appearing quite simple and others joining into intricate colonies or filaments.
Phytoplankton can rapidly grow or disappear based on many factors, such as temperature, nutrients, mixing of the water, and predation. By regular tracking of the dominant groups of phytoplankton, we can see their changes in response to natural and human-caused changes in the environment.
Phytoplankton Monitoring Project Goal: To monitor 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.
How we Monitor: Each week, water samples from two locations at the lake are gathered and analyzed by a volunteer citizen scientist with a microscope. The phytoplankton are identified and counted to get an idea of the currently common types.
Most common phytoplankton in Green Lake
Samples from April 29, 2025
Tracking Green Lake phytoplankton over time at North Dock
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 succession in Green lake. North Dock 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 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.
What about elsewhere in the lake? Phytoplankton from the water at Duck Island Launch showed similar successions. The difference is, this site is a protected cove and provides a slightly better habitat or collection area for cyanobacteria. Look for the orange and turquoise-coded peaks of cyanobacteria called Dolichospermum and Aphanizomenon, respectively.
You might remember seeing widespread green or yellowish scum in Green Lake last winter. This corresponded to the turquoise Aphanizomenon peak in the graph above. Even though Aphanizomenon does not normally make toxin, this was a time to stay out of the water, just to be safe.
In the Fall of 2024, Green Lake also had a limited bloom of Dolichospermum (orange peaks in the graph above), but this did not result in accumulated scum along the shore. In addition, though Dolichospermum does often make toxin, this time we were spared.