BIOTIC AND ABIOTIC INTERACTIONS BETWEEN SURFACE AND INTERSTITIAL SYSTEMS IN RIVERS

Water chemistry and community assemblages of surface and interstitial invertebrates were studied at seven sites on the French Rivers Rhone and Ain at surface and at 50 cm depth into the bed sediments. Chemical factor: allowed differentiation of surface water from groundwater and detection of water exchanges defining clear downwelling and upwelling zones. At some sites, interstitial water showed both surface and phreatic conditions, characterizing the underflows of the Rhone or of the Ain. In the interstitial area, most taxa showed no significant correlations with water chemistry. Some epigean and hypogean fauna showed correlations with certain factors. Clear relationships appeared between the water exchanges and the distribution of surface and interstitial faunas. In interstitial samples, epigean and some hypogean species characterized the downwelling zones while others seemed to be strictly linked to the upwelling zones. In surface samples. the presence of hypogean species was associated with regions of groundwater upwelling.