Stream order-dependent diversity metrics of epilithic diatom assemblages

Diatoms are considered as an appropriate indicator group for ecological status assessment of surface waters. These organisms can be indicative not only of the waterchemical but also of the hydro-morphological characteristics (e.g., stream size, physical habitat diversity) of running waters. In this study, diatom diversity metrics (species number, Shannon diversity, and evenness) from 506 sites in Pannon ecoregion (Hungary) were compared to the Strahler stream order system established with ArcView GIS 3.2. SOM analyses were performed to exclude the effect of nutrients on diversity metrics along the stream orders. Mixed-effects linear models and Tukey's post hoc test revealed a linear relationships between species number, diversity and stream orders on ecoregion level from first- to eighth-order streams. The species number increases with an average of 8%, and the diversity by 10% per unit increase of the stream order. However, we could not find relationships with evenness. Autotrophic diversity metrics based on diatom species data appear to increase parallel with the stream order while those of heterotrophic metrics (published in the literature) maximize at medium stream orders. We argue that stream order is a relevant typological parameter which can basically determine the diatom diversity metrics, and it is well applicable in biomonitoring.