Stone crayfish in the Czech Republic: how does its population density depend on basic chemical and physical properties of water? L’écrevisse des torrents en République tchèque: comment la densité de sa population dépend des propriétés physico-chimiques de l’eau ?

The stone crayfish (Austropotamobius torrentium Schrank) is one of the two native crayfish species in the Czech Republic. The populations as well as physical and chemical parameters of water (pH, conductivity, dissolved oxygen, undissolved particles, NH3, NH4+, NO2 , NO3 , phosphorus, Ca2+ and SO42 ) of 33 streams were examined to find the ecological plasticity of this crayfish and some relations between these parameters and population densities. The mentioned parameters often significantly varied at the sites. Two approaches were applied to find relations between these parameters and observed abundance. At first, the observed streams were compared using RDA (streams × physical-chemical parameters). No significance was found while testing relationship between the streams grouped along the 1st axis of model and the observed abundances of stone crayfish. However, some correlations between abundance and conductivity, calcium, nitrates and sulphates were found using polynomial regression. These relationships are explicable in terms of mutual correlations, underlying geology and other factors which affect abundances. In conclusion, A. torrentium is able to inhabit waters with a large range of physical and chemical parameters of the water without any fundamental influence on population densities. Water properties play an indisputable role as limiting ecological factors at uncommon concentrations, but population densities are probably influenced much more by the types of habitats, habitat features, predation and other ecological factors. L’écrevisse des torrents (Austropotamobius torrentium Schrank) est l’une des deux espèces d’écrevisses indigènes de la République tchèque. Les populations ainsi que les paramètres physiques et chimiques de l’eau (pH, conductivité, oxygène dissous, particules en suspension, NH3, NH4+, NO2 , NO3 , le phosphore, Ca2+, SO42 ) de 33 cours d’eau ont été examinés pour déterminer la plasticité écologique de cette écrevisse et des relations entre ces paramètres et les densités de population. Les paramètres étudiés varient souvent beaucoup suivant les sites. Deux approches ont été appliquées pour chercher des relations entre ces paramètres et l’abondance observée. Dans un premier temps, les rivières étudiées ont été comparées par une RDA (ruisseaux × paramètres physico-chimiques). Aucune signification n’a été trouvée en testant la relation entre les rivières regroupées le long du 1er axe de ce modèle et les abondances observées des écrevisses des torrents. Cependant, certaines corrélations entre l’abondance et la conductivité, le