Contribution to the knowledge of low altitude mediterranean rivers: the hydrographic system of the River Arc (Bouches-du-Rhone, France). Benthic invertebrates overview

The Arc river's hydrographic system is situated in a heavily urbanised and industrialized area where agriculture nevertheless still plays an important role. This paper presents an overall view of the water quality and the benthic populations based on a multivariate analysis of an annual cycle (18 physico-chemical parameters, 17 study sites : 9 on the river's main course, 8 on its tributaries). The resultant hydrochemical states are arranged in order of an eutrophication gradient represented by the first principal component. The allogenic elements, which have a preponderant role in the hydrosystem's activity, cause man-made eutrophication which overshadows the river's natural eutrophication. Only a few unpolluted tributaries still remain close to their original state. The benthic populations of the Arc hydrographic network were studied through a correspondence analysis using 15 sampling points containing 151 taxa selected on the axial stream and its tributaries. The upstream stations can be characterized by the presence of lenitophilous species adapted to the transitory environment. Throughout most of the river's course, a high level of organic and chemical pollution has helped to proliferate saprobiont species. In the little or undisturbed tributaries, there is a population which can be basically categorized as a rhithron biocenotype containing species capable of recolonizing the course of the main river.