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Research article2016Peer reviewed

Effects of counting variances on water quality assessments: implications from four benthic diatom samples, each counted by 40 diatomists

Werner, Petra; Adler, Sven; Dressler, Mirko


The European Water Framework Directive (EU WFD) aims to improve and maintain the water quality of lakes and rivers. Diatoms play an important role for implementing the EU WFD as they are widely used to assess water quality. To asses and use diatom-based evaluations in practice, it is essential to know and to minimize the analyst-dependent variability of the primary diatom results. We compare the counting results of benthic diatoms from two river and two lake samples as identified by 40 participants of the first German benthic diatom intercalibration exercise. Differences among participants and auditors are calculated with the Bray-Curtis distance, and similarities are graphically displayed by detrended correspondence analyses. Additionally, this study identifies the effects of counting variances on the ecological water assessment with the German PHYLIB method for implementing the EU WFD using the above dataset. Counting result differences among participants may have a significant impact on the assessed water quality. Some taxonomically problematic taxa do not have indicator values (no impact), sometimes, several taxonomic mistakes counterbalance each other during assessment (no or minor impact), and sometimes, taxonomic differences greatly influence the assessment due to highly deviating taxon abundances and/or strongly differing indicator values between mistaken taxa. Intercalibration exercises promote discussion about taxonomy, identify problematic taxa, and harmonize taxonomic concepts among diatomists. Thus, the shown analyst-induced variability of diatom counts may be reduced with an intercalibration exercise, thereby further refining the accuracy of water quality assessments.


Intercalibration exercise; Biological monitoring; Quality assurance; Lakes; Rivers; EU Water Framework Directive

Published in

Journal of Applied Phycology
2016, Volume: 28, number: 4, pages: 2287-2297
Publisher: SPRINGER

    Sustainable Development Goals

    SDG6 Clean water and sanitation

    UKÄ Subject classification

    Environmental Sciences

    Publication identifier


    Permanent link to this page (URI)