Chlorophyll in various forms is bound within living cells of photosynthetic organisms, such as phytoplankton and cyanobacteria (blue-green algae). The amount of chlorophyll found in a water sample is used as a measure of the concentration of phytoplankton. These measurements contribute to the understanding of the general biological "health" of the system, such as its trophic status or primary production. Chlorophyll measurements can also identify algal bloom events and their effects on water quality and anticipate toxic algal blooms.
Chlorophyll fluoresces when irradiated with light of a particular wavelength (435-470 nm). For field measurements, in situ fluorometers induce chlorophyll to fluoresce by shining a beam of light of the proper wavelength into the water and then measuring the higher wavelength light which is emitted. These real-time chlorophyll measurements complement extractive lab analysis.