Diatoms in water quality assessment: to count or not to count them?

Two methods of quantifying diatoms were compared: (i) by counting valves, and (ii) by simple assignment of individual species to a seven-point scale according to their coverage on a glass slide. The saprobity-trophic index introduced in the Czech Republic for assessing the ecological status of running waters in accordance with the requirements of the Water Framework Directive (WFD) was used for comparison. There were no statistically significant differences between the indices as calculated by the studied methods, and therefore the comparable indicative values of the two approaches were demonstrated. In all cases, however, indices based on coverage classes achieved higher correlation with the fundamental water chemistry parameters than did indices based on diatom valve counts. To explain the cause of higher correlations, different variants of editing input data were used. The results support the assumption that the main cause of the poorer predictive value for the index based on diatom valve counts is that the abundant taxa are attributed too much importance in the final assessment of the environment quality. Using degrees of coverage for the quantification of diatoms in evaluating the ecological status thus appears to be preferable (or at least sufficient) for the given purpose.