Chlorophyll a

Chlorophyll a is the green pigment in photosynthesizing algae that helps them produce food. Measuring chlorophyll is an indicator of the amount of algae in the water, which uses both nitrogen and phosphorus to grow. Too much algae in the water reduces the oxygen available for other organisms, and some algal blooms produce toxins that are harmful to all living things.

How is it scored?

Chlorophyll a (Chla) was used as an indicator in lake analysis. Data from April 1–September 30, 2018 were analyzed, representing the algae growing season. For each Chla sample, the measurement was compared to the threshold on a pass/fail basis. Site scores were averaged into an overall region chlorophyll a score.

The chlorophyll a threshold used for the western Lake Erie basin analysis was the Great Lakes Water Quality Agreement Annex 4 – nutrients threshold 0.1 mg/L.

A close-up of an algal bloom in Lake Erie. Photo by Jeff Reutter via Flickr, CC BY.