Chlorophyll a

Chlorophyll a is the most important pigment involved in energy capture from light during photosynthesis, and all photosynthetic plants, including algae and cyanobacteria, contain it. Consequently, this pigment is the most widely used metric for algal biomass in surface waters.

“Algae” is a general and variably applied term describing a highly diverse group of mostly microscopic organisms. Algae are distinct from other aquatic photosynthetic organisms that use chlorophyll as their primary photosynthetic pigment in that they lack specialized structures, such as roots and leaves, found in terrestrial plants and macrophytes. Algae are the mostly widely distributed group of aquatic photosynthetic organisms, and they typically are the most important base of aquatic food webs and nutrient cycles. The abundance and species composition of algae in water bodies is a key determinant of water quality. 

Photosynthetic organisms such as algae use pigments to convert sunlight into energy. The pigments absorb energy from sunlight preferentially at specific wavelengths, reflecting light that is not absorbed. The differential absorption and reflection of specific wavelengths of light is the basis of both laboratory and remote sensing techniques to measure chlorophyll (and hence algal biomass) in aquatic ecosystems.

Chlorophyll a may be measured by various techniques. Most often, algae suspended in a water sample is collected by filtration, the pigment is extracted with a solvent, and it is quantified in the lab by spectrophotometry. Remote sensing methods measure light reflected from water surfaces using wavelengths characteristic of the absorbance spectrum of chlorophyll to infer levels of algal biomass.

Remote sensing methods used to measure water clarity also can provide information on algal biomass because water clarity often is negatively correlated with biomass, i.e., increasing algal biomass causes decreasing water clarity, if other substances that affect clarity (see CDOM and turbidity) are not present at high levels.