Determinants of chironomid species richness in mid-European temperate rivers – environmental factors, regional influences, diversity, and seasons

Determinants of riverine chironomid species richness were assessed in seven rivers in an area representative of the European temperate climatic zone. The impact of environmental factors, seasonality and regional species richness (RSR) on local species richness (LSR) as well as the exchange of species between seasons and the correlations between gamma diversity (i.e. RSR), alpha diversity (i.e. LSR), and beta diversity, i.e. the relationship between RSR and LSR, were studied. A stepwise forward regression model with RSR, and a similar model with no RSR explained two thirds of LSR variability each, and seasonality (summer) was significant only in the former model. RSR accounted for 83% of explained LSR variability in the latter model. RSR differed both from river to river and from season to season, being lowest in autumn. Winter was most distant from all other seasons as regards species composition of assemblages (UPGMA clustering). Almost all correlations between diversity measures, i.e. gamma (RSR), setwise and pairwise beta, and alpha (LSR), were significant only in autumn, indicating greater assemblage stability in autumn than in other seasons. As no significant environmental factor differed between seasons, the stability seems related to undisclosed biotic mechanisms.