Basic Concepts and Key Parameters of Chlorophyll Fluorescence

Photosynthetic rates can be measured in several different ways. Classically, phytoplankton productivity has been measured by uptake rates of¹⁴C-labelled inorganic carbon (Ci, i.e. CO₂ and HCO₃^-), which is a sensitive measure even for sparse phytoplankton assemblages. For macrophytes, gas exchange of CO₂ or O₂ have been used as preferred methods of photosynthetic productivity. All these above mentioned methods have the disadvantage of being slow (minutes to hours or days) and the need for enclosing the plants, the latter of which invariably alters their surroundings from natural ones in terms of light (both irradiance and spectrum), water flow and decreasing nutrient, such as Ci, levels, as well as increasing O₂ concentrations, which may impede on rates. For the last 20 years, chlorophyll fluorescence measurements have increasingly replaced the other methods as being both sensitive and speedy, and applicable under natural, non-enclosed, conditions. This chapter explains the background to chlorophyll fluorescence, as well as the principles of pulse-amplitude modulated (PAM) fluorometry and, briefly, fast repetition rate fluorometry (FRRF) measurements.