Measurement
Chlorophyll-a
A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.
Chlorophyll-a is the pigment that makes algae green; measuring it gives a direct estimate of algal biomass in the water column. High chlorophyll-a numbers mean active algal communities, often visible as cloudy or scummy water. Chlorophyll-a is one of the three core inputs to LakeGrade. Cutoffs: below 5 µg/L = A, below 10 µg/L = B, below 20 µg/L = C, below 30 µg/L = D, above 30 µg/L = F.
The per-lake pages on LakeQuality always show the specific Chlorophyll-a value for that lake, so you can move from the general concept to the specific lake without leaving the site.
What It Means for Your Lake
Chlorophyll-a (Chl-a) is the primary photosynthetic pigment in algae and cyanobacteria. When measured in lake water samples, it provides a direct estimate of algal biomass, how much algae is actually growing in the lake at the time of sampling. Chlorophyll-a is measured in micrograms per liter (ug/L) from water samples collected in the upper mixed layer (typically the top 2 meters) and analyzed in a laboratory using spectrophotometry or fluorometry. In Minnesota and Wisconsin lakes, chlorophyll-a concentrations range from less than 2 ug/L in the clearest oligotrophic lakes to over 100 ug/L during severe algae blooms. Values below 5 ug/L indicate excellent water quality with minimal visible algae. Between 10 and 20 ug/L, the water takes on a greenish tint and algae becomes noticeable. Above 20 ug/L, nuisance conditions develop with visible surface scums, reduced clarity, and increased risk of harmful cyanobacteria toxins. LakeQuality weights chlorophyll-a at 20% of the overall grade because it directly measures the biological response to nutrient loading. Unlike phosphorus, which measures potential for algae growth, chlorophyll-a measures actual algae present. Chlorophyll-a levels vary substantially within a single summer season, so median summer values are used for grading.
Frequently Asked Questions
What is chlorophyll-a?
A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.
Why does chlorophyll-a matter for lake health?
Chlorophyll-a (Chl-a) is the primary photosynthetic pigment in algae and cyanobacteria. When measured in lake water samples, it provides a direct estimate of algal biomass, how much algae is actually growing in the lake at the time of sampling. Chlorophyll-a is measured in micrograms per liter (ug/L...
Related Terms
Phosphorus
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
Algal Bloom
A rapid increase in algae population in a lake, often visible as green discoloration or surface scums, triggered by warm temperatures and excess nutrients.
Cyanobacteria
Photosynthetic bacteria (commonly called blue-green algae) that can form harmful blooms and produce toxins dangerous to humans and animals.
Trophic State Index
The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.
Source: EPA National Aquatic Resource Surveys, 2026.