The use of spectral fluorescence methods to detect changes in the phytoplankton community

Abstract

In vivo fluorescence methods are efficient tools for studying the seasonal and spatial dynamics of phytoplankton. Traditionally the measurements are made using single excitation-emission wavelength combination. During a cruise in the Gulf of Riga (Baltic Sea) we supplemented this technique by measuring the spectral fluorescence signal (SFS) and fixed wavelength fluorescence intensities at the excitation maxima of main accessory pigments. These methods allowed the rapid collection of quantitative fluorescence data and chemotaxonomic diagnostics of the phytoplankton community. The chlorophyll a-specific fluorescence intensities (R) and the spectral fluorescence fingerprints were analysed together with concentrations of chlorophyll a in different algal size-groups, phytoplankton biomass and taxonomic position. The lower level of R in the southern gulf was related to the higher proportion of cyanobacteria relative to total biomass and the lower abundance of small algae. The phycoerythrin fluorescence signal was obviously due to the large cyanobacteria. The basin-wide shift in the shape of chlorophyll a excitation spectra was caused by the variable proportions of differently pigmented cyanobacteria, diatoms and cryptomonads.