The Hydraenidae and Elmidae assemblages living in the Órbigo River Basin (NW Spain) were studied during one year. The aim of the research was to determine which factors were best related to species composition. This knowledge is the first step towards the definition of indicator species or assemblages.
Canonical Correspondence Analysis (CCA) showed that altitudinal gradient was the factor most correlated with beetle distribution. However, some other variables, such as water mineralization and eutrophication, were also important.
Using TWINSPAN program, groups of sites were defined and, afterwards, represented on the CCA diagram. Several species assemblages were defined on the basis of their frequencies of occurrence in these site groups. The environmental features of site groups and beetle assemblages were assessed with the aid of CCA. In this way, assemblages typical of high reaches could be separated from those of low stretches of the rivers. Similarly, communities from non-polluted waters could also be defined. Although several species are present in polluted sites, no assemblage exclusive to these sites has been found.