The key takeaways from the 2025 monitoring season are: • In 2025, the TSI values were in the eutrophic range. Eutrophic lakes (TSI >50) have less clear water and high nutrient concentrations and algal growth. • Monitoring on Green Lake began in 2005, a year after an alum treatment. • Nitrogen-to-phosphorus (N:P) ratios were below 25 throughout the monitoring season. This indicates the potential for algal blooms to be dominated by cyanobacteria (which have the ability to produce toxins). • Long-term trends suggest that water quality in Green Lake has been declining over time, with increasing chlorophyll, nitrogen, and phosphorus concentrations. These trends are seen even with two alum treatments (2004 and 2016) since monitoring within the Lake Stewardship Monitoring Program began, in 2005. • Numerous samples were submitted for toxin testing in 2025 around Green Lake. In June, September and October, toxin testing found elevated microcystin concentrations above the Washington State
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The average of these three TSI indicators during the growing season can be used to place lakes in one of three broad categories: <40 = oligotrophic (low productivity), 40 to 50 = mesotrophic (moderate productivity) >50 = eutrophic (highly productive).
