A fluorometer is used to estimate the amount of phytoplankton (plant-like, microscopic organisms) in the water. This is typically done by measuring the primary photosynthetic pigment in phytoplankton called chlorophyll a.
Why we collect this data
Phytoplankton are primary producers as well as the base of the food chain. By collecting information about the phytoplankton, we can learn more about the ecosystem.
How it works
When chlorophyll is irradiated with light, it emits (sends out) light of a higher or lower wavelength. This is called fluorescence, the absorption of light energy at one wavelength and its re-emission at another. A fluorometer measures fluorescence. It sends out a certain wavelength of light and measures the the intensity of light that is returned when it is re-emitted by the phytoplankton. The greater the intensity of light that the fluorometer reads, the more chlorophyll and phytoplankton in the water.
Image credit: http://www.turnerdesigns.com/products/laboratory-fluorometer/10au-laboratory-fluorometer