Original paper

Utility of a single a priori river typology for reference conditions of boreal macroinvertebrates and diatoms

Mykrä, Heikki; Aroviita, Jukka Hämäläinen; Karjalainen, Satu; Maaria Visuri, Mika; Riihimäki, Juha; Miettinen, Juha; Vuori, Kari-Matti


The ecological quality of river habitats is increasingly assessed using multiple taxonomic groups. As widely recognised, assessments should be based on the reference condition approach where potentially impaired sites are compared to pristine or near pristine reference sites. To enable valid comparisons, natural variability of habitat characteristics and biotic assemblages should be controlled for, by using either an a priori river typology as suggested by the European Union Water Framework Directive (WFD) or by some other reference framework. We examined a simple WFD river typology, based on two broad-scale ecoregions, catchment area, and geology, as a basis for reference conditions of macroinvertebrates and diatoms in Finland. The ability of this a priori typology to account for natural variation in assemblage composition was evaluated by comparing it with null models (no typology) and with a posteriori clusters ('optimal' biological typology). The biological clusters were clearly more effective than the WFD typology in partitioning assemblage variation for diatoms, whereas differences among the groupings were less evident for macroinvertebrates. Similarly, the number of observed versus expected type-specific taxa (O/E-ratio) was more precise with the a posteriori clusters than with the WFD typology for diatoms, while there were no clear differences among the groupings for macroinvertebrates. For diatoms, the typology was virtually inefficient in improving the precision of O/E from that of the null model. Strong biogeographical patterns in assemblage composition of both biotic groups were evident between and within the two ecoregions. Variation in macroinvertebrate assemblages was primarily related to geographical location and river size, whereas geographical location and water chemistry were the most important environmental factors for diatoms. Our results suggest that simple a priori typologies could be used as a framework for reference condition in freshwater bioassessment. However, when assessments are conducted using multiple taxonomic groups, typologies should be based on factors important for each individual group.


assemblage structurebioassessmentbiological quality elementsreference condition approachwfd