Analysis of Phytoplankton Communities Using Hyperspectral Absorption Measurements

Methods based on optical measurements are well suited for phytoplankton observation. They are rapid, require comparably low effort, and are often automatable. Besides fluorescence, also the absorption of the water is influenced by the concentration and type of the phytoplankton present.

With instruments based on integrating cavities like the point source integrating cavity absorption meter (PSICAM), it is possible to analyze natural water samples in situ. Due to its design, the PSICAM has a long optical path length and the sample has not to be concentrated on a filter prior to analysis. Additionally, light scattered by particles present in the sample has no effect on the absorption measurements. Since the hyperspectral measurements are of high quality, efforts have been made to adapt this approach also for continuous and automated measurement (flow-through-PSICAM).

Absorption coefficient data were obtained using both instruments on research cruises in the German Bight and along the Norwegian coast. In case of the discrete measurements, the water samples were filtered through various pore sizes to separate the bulk absorption into the absorption of the different phytoplankton size classes (micro-, nano-, and picophytoplankton). A proxy for chl-a concentration was derived from the data which was used to characterize phytoplankton biomass distributions in the area of investigation. Furthermore, in order to obtain additional information about the phytoplankton community beyond biomass distributions, also the spectral shape of the absorption spectra was analyzed. Attempts were made to attribute different spectral shapes to different phytoplankton groups and find similarities or differences between the cruises stations, respectively.