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.
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 the condition of the lake water. Lake temperature, nutrient levels, sunlight, and amounts of predatory zooplankton are all important factors for the survival of each type of phytoplankton. 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 and less often during the rest of the year (See the Water Quality page for more details). Unlike E.coli and most other bacteria, our cyanobacteria of interest are large enough to see easily in a standard microscope. See below for current microscope counts.
What is the microscope telling us about Green Lake this week?
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. A couple of drops of this scum under the microscope revealed a large amount of a coiled species of Dolichospermum cyanobacteria, accompanied by lesser amounts of a straight-chain Dolichospermum and Microcystis. Recent routine testing at King County Environmental Laboratory did show elevated levels of cyanotoxin in the lake, presumably due to our friends in the pictures above. Stay out of the water for now! Link to King County Beach Closures
Meanwhile, other nontoxic types of phytoplankton are still thriving in Green Lake. Below are the currently most abundant types. See below * for more about how these data are obtained.
last updated 9/10/25
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 gathered, analyzed and photographed by a volunteer scientist with a microscope. The phytoplankton are identified and counted to get an idea of the currently common types.
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