Trichoptera and ephemeroptera as indicators of environmental changes of the Rhone river at lyons over the last twenty-five years

Systematic light trapping was carried out in Lyon on the banks of the Rhǒne river from 1958 to 1982, mainly in the months of June and July. The changes in the type and abundance of the adult Trichoptera and Ephemeroptera species captured over the last 25 years were studied in relation to the perturbations sustained by the river‐and thus by the larval instars. A Correspondence Analysis carried out on the monthly samples defined a first factorial axis which could represent a ‘biological time’ scale characterized by phases of rapid (1959‐1974) or slower (1974‐1982) change of the faunal communities. Five ‘successive’ faunal groups were defined according to the length of historical presence of each of the 88 species along this axis. The most stenoecic species (faunal groups 1, 2 and then 3) seem to disappear and gradually only a small number of species whose ecological requirements are less demanding (faunal groups 4 and 5) subsist. This faunal impoverishment is a consequence of the increasing uniformity of the biotope caused by continuous environmental changes mainly in flow regulation. This has led to reduction in diversity of the substratum and possible food sources. Regular flush‐outs of upstream reservoirs release sediments which clog interstitial spaces over long periods and this effect has been aggravated since 1966 by the bringing into service of a dam situated just downstream from the station. It seems that a new but delicate equilibrium has been reached since 1974; this situation probably corresponds to a less stenoecic biotic community.