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Aquatic Microbial Ecology

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AME 18:95-103 (1999)  -  doi:10.3354/ame018095

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

Gerald Pfister1,2,*, Bettina Sonntag2, Thomas Posch2

1Institute of Limnology, Austrian Academy of Sciences, Gaisberg 116, A-5310 Mondsee, Austria
2Institute of Zoology and Limnology, University of Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria

ABSTRACT: To compare the efficiency of 2 common methods in protozooplankton ecology the protozooplankton spring community of a mesotrophic prealpine lake was investigated. A direct live count and a routinely applicable modification of a quantitative protargol stain (QPS) were evaluated with respect to their reliability in determining and quantifying pelagic freshwater ciliates and heterotrophic and mixotrophic flagellates. Cell counts of most ciliate species from field samples as well as of cultures were not significantly different when estimated by the 2 methods. The QPS allowed for detailed ciliate identification even for species not determinable by the applied direct live observation. Consequently, 60% of all determined species from field samples were identified exclusively by means of the QPS. Both flagellate and ciliate cell volumes showed high and species-specific shrinkage after the QPS, thus underestimating calculations of cell volumes. After the QPS, aloricate ciliates shrank to less than 20% of their respective live volumes, whereas the 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 counted by the QPS method were 37% of the direct live counts. Based on these results the advantages and disadvantages of the 2 tested methods used in protozooplankton analysis are summarized.

KEY WORDS: Ciliates · Fixation · Flagellates · Shrinkage · Quantification

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