A comparison of ecological optima of soft-bodied benthic algae in Norwegian and Austrian rivers and consequences for river monitoring in Europe.

Alpine and Nordic rivers are often considered as being among the most pristine in Europe. Nevertheless, acidification and eutrophication impact surface waters in these regions. Soft-bodied, i.e. non-diatom, benthic algae are used as indicators for eutrophication and acidification in both Norway and Austria, but consistency of indicator values has never been tested. We compared species optima with respect to pH, conductivity, total phosphorus (TP), and NO₃(-)-N concentration for 21 species, derived from geographically and temporally extensive datasets from Norway and Austria, respectively. The ranges of all four measured parameters were different between Norway and Austria, with Austria having generally higher values for all measured parameters. Optima for all 21 species with respect to pH, conductivity and NO₃(-)-N were significantly different between Norway and Austria, while 5 of the 21 taxa showed no significant differences for TP. Nevertheless, species optima for Norway and Austria were significantly correlated with each other for TP, pH and conductivity. This indicates that positions of species optima relative to each other may be stable across ecoregions, in spite of the absolute values of species optima being different. In contrast, optima with respect to NO₃(-)-N were not correlated, possibly suggesting a lesser importance of NO₃(-) in shaping benthic algal assemblages than TP and pH. We conclude that the use of eutrophication and acidification models across different ecoregions may give meaningful results, but requires regional testing of species optima.