Comparison of fluorometric and microscopical quantification of phytoplankton in a drinking water reservoir by a one-season monitoring program

Abstract

Phytoplankton are key components in aquatic ecosystems and therefore an important target of monitoring analyses. Microscopical counting, although providing most detailed results, is time and labor intensive and requires highly skilled analysts. In situ spectral fluorescence measurements provide a much faster analysis with a higher spatiotemporal resolution. A one-season survey of phytoplankton assemblages was performed in order to compare the results of the spectrofluorometric measurements to classical microscopical determination in different sections of a drinking water reservoir. The investigations were performed with the spectrofluorometer FluoroProbe (FP) by bbe Moldaenke GmbH (Kiel, Germany), which is designed to discriminate among diatoms, green algae, Cryptophyta and cyanobacteria. The results of phytoplankton quantification as revealed by total chlorophyll a (Chla) measurements with the FP and total biovolumes determined by microscopy showed a good correlation.The accordance between the two approaches was best for diatoms and much lower for the other spectral groups. The proportion of green algae was generally overestimated by FP measurements in comparison to biovolumes. Contrary, the percentage of cyanobacteria was often underestimated by FP compared to microscopical analyses. A clear underestimation of cyanobacteria by FP measurements even at high abundances of Microcystis sp. was observed in two samples. No influence of species composition on the congruence between microscopical analyses and FP measurements was detected.