Comparison of a direct live count and an improved quantitative protargol stain (QPS) in determining abundance and cell volumes of pelagic freshwater protozoa

To compare the efficiency of 2 common methods in protozooplankton ecology t he protozooplankton spring community of a mesotrophic prealpine lake was in vestigated. A direct live count and a routinely applicable modification of a quantitative protargol stain (QPS) were evaluated with respect to their r eliability in determining and quantifying pelagic freshwater ciliates and h eterotrophic and mixotrophic flagellates. Cell counts of most ciliate speci es from field samples as well as of cultures were not significantly differe nt when estimated by the 2 methods. The QPS allowed for detailed ciliate id entification even for species not determinable by the applied direct live o bservation. Consequently, 60% of all determined species from field samples were identified exclusively by means of the QPS. Both flagellate and ciliat e cell volumes showed high and species-specific shrinkage after the QPS, th us underestimating calculations of cell volumes. After the QPS, aloricate c iliates shrank to less than 20% of their respective live volumes, whereas t he armoured Coleps spetai did not shrink significantly. Thus, drawbacks in applying average conversion factors for cell volume calculations after the QPS are discussed. Total heterotrophic and mixotrophic flagellate numbers c ounted by the QPS method were 37 % of the direct live counts. Based on thes e results the advantages and disadvantages of the 2 tested methods used in protozooplankton analysis are summarized.