Measurement

Chlorophyll-a

A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.

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