Chlorophyll a is key to photosynthesis – the conversion of sunlight and Carbon Dioxide (CO2) into Oxygen (O2) and simple sugars – the universal energy currency of cells. It is therefore present in all in photosynthetic cells: plants, phytoplankton and cyanobacteria (blue-green algae).
Chlorophyll absorbs strongly in the blue and red regions of the electromagnetic spectrum, accounting for its pronounced green colouration. Most of the absorbed light is converted to sugars during photosynthesis, though a small fraction is lost as fluorescence, the re-emission of absorbed light at longer wavelengths – around 690 nm (deep red). The amount of fluorescence varies depending on physiological factors – light intensity, temperature, “stress” etc.
A Chlorophyll fluorometer shines a beam of blue light into the water and measures the amount of red light that returns. This is used as a proxy for the amount of chlorophyll in a water sample and the concentration of phytoplankton.
Chlorophyll levels determine primary production – the base of the aquatic food web, and can also be used identify algal bloom events which can negatively affect water quality (Harmful Algal Blooms HAB or e.g. Red Tides).