Variabilität von Makrozoobenthosproben und Bewertungsergebnissen der Fließgewässer vor dem Hintergrund der Umsetzung der EG-Wasserrahmenrichtlinie

The EU Water Framework Directive (WFD) demands an integrated biological assessment of surface water bodies. The assessment shall be based on “Biological Quality Elements” and is performed by comparing the observed status with type specific reference conditions reflecting the impact of various stressors. Moreover the WFD demands restoration measures whenever the ecological status is “moderate” (3) or worse. This implies that there is no need for restoration measures if the status is “good” (2) or better. With regard to this fact, it is necessary to know the degree of uncertainty of the assessment results to be able to estimate whether an expensive restoration of a stream (or waterbody) is justified or not. The degree of uncertainty is linked to the variability of the data. Thus the aim of the first part of the present study was to identify different causes of variability of macroinvertebrate samples and moreover to quantify the impact of the variability on assessment results. The results of the study stress the importance of implementing standardized sampling and sorting protocols and quality control mechanisms in macroinvertebrate assessment schemes. In the second part of the study a multimetric assessment system for small streams in the younger moraines of the alpine foothills was developed. This system reflects the impact of various stressors, including hydromorphological degradation, stagnation, residual flow, land use in the catchment, pesticides and hormonequivalent substance input. However, usually hydromorphological degradation is most important. The assessment system is composed of six different “core metrics“ which have been selected following three predefined principles: 1. As many metrics as necessary to attain a robust result and to enable a profound data interpretation; as few metrics as possible to guarantee simple applicability. 2. Stream type-specific deviations are necessary; however, the approaches for the individual stream types should be as similar as possible. 3. All assessment criteria of the WFD should be covered: composition and abundance, share of tolerant taxa and degree of diversity. Special interest the study was to calculate the degree of probability of the assessment results. It was recognized that the high variance of the data lead to results which are not interpretable with respect to the ecological status of the waterbody. At least four of the five different ecological status classes could be achieved with a certain probability. In order to minimize the variability of the data, a simple transformation of the metric values was performed. After the transformation the probabilities of the assessment results were distributed more distinctively and thus delivered interpretable results. Based on these better results a justification of restoration measures is possible.