LONGITUDINAL CHANGES IN TRICHOPTERA AND COLEOPTERA ASSEMBLAGES AND ENVIRONMENTAL-CONDITIONS IN THE LOIRE RIVER (FRANCE)

A survey of benthic invertebrate fauna in 38 sites along 800 km of the Loire River from the spring to 200 km upstream of the mouth was carri ed out. A classification analysis (UPGMA) of Trichoptera and Coleopter a (31 species) was made in order to define the main changes and transi tion zones in their longitudinal assemblages. A variety of environment al data (physico-chemical factors and hydrology) provided by the Agenc e de I'eau were used to define possible relationships between faunisti c changes and environmental parameters. It was concluded that faunisti c changes concerning the upper reach of the Loire River could be compa red to those of a small river. From 100 to 800 km, the enhancement of discharge is a major environmental component but no link was establish ed with the faunistic changes. The species richness is higher in the u pper reaches than in the lower reaches although disturbance related to hydrological variations (floods and droughts) is higher in the upper reaches than in the lower reaches. The effects of polluted areas and/o r dams do not drastically modify Trichoptera and Coleoptera assemblage s, even if the species richness drops downstream of Villerest, the maj or dam. One hundred kilometres downstream, two transition zones can be defined, the first between 350-400 km, the last 800 km downstream. Be tween 350 and 800 km the fauna is rather homogeneous, however new spec ies appear 800 km downstream. The transition zones seem essentially re lated to major changes of stream gradient.