Spatial congruency of benthic diatom, invertebrate, macrophyte, and fish assemblages in European streams

Understanding patterns in taxonomic congruency and the importance of environmental variables as drivers of these patterns is a prerequisite for analyzing changes in regional species pools. Here we evaluated the spatial correspondence of benthic diatoms, invertebrates, macrophytes, and fish assemblages in minimally disturbed lowland (n=38) and mountain (n=32) streams and, using constrained ordination and variance decomposition, determined how environmental and biological variables related to these patterns. Using different measures of diversity, abundance, and community composition, principal components analysis failed to reveal a single gradient in lowland and mountain streams. This was due to differences in the relationships between individual diversity measures and environmental variables characterizing habitat diversity (number of debris dams, silt and cobble substratum) in lowland streams, and catchment area and oxygen saturation in mountain streams. Despite the low spatial coherence of individual measures, integral multivariate analyses of community structure revealed coherent patterns between taxonomic groups. From 31.0% to 47.7% of the variance in diversity of fish, invertebrate, and macrophyte assemblages in mountain streams and from 11.9% to 33.7% of the variance in diversity in lowland streams were explained by one or more of the remaining three groups. Partial redundancy analyses showed that composition of other taxonomic groups was generally a better predictor of assemblage diversity than interactions with environmental variables, lending support to the importance of species interactions for spatial coherence. Finally, although we found little support for a single biodiversity gradient, correlations between fish diversity and macrophyte and invertebrate diversity in mountain streams and between macrophyte diversity and invertebrate and diatom diversity in lowland streams suggest that these two groups (i.e., fish in mountain and macrophytes in lowland streams) may function as surrogate indicators for the diversity of other taxonomic groups.