Simple and sensitive in situ algae fluorescence sensor based on fibre optics

Monitoring of biological activity in natural waters has become a task of increasing importance. In particular, mapping of microscopic, free drifting algae and phytoplankton is important. One method of determining the density of phytoplankton is based on measurements of chlorophyll-¿¿ fluorescence. Fluorescence from chlorophyll-¿¿ in living plants is a very complex process closely linked to the photosynthesis. The fluorescence yield is a parameter dependent on a number of highly variable factors in the plant itself and its environment: factors like illumination (adaption), nutrient supply, pollutants and the age of the plant. The excitation spectrum basically characterises the relative contents of primary absorbing pigments, and may therefore be used to indicate specific algae species or groups. This paper describes a fluorescence sensor based on fibre optics. Its main advantage is that the probe itself is passive and may be operated with a number of different source/detector combinations.