Long-term Changes and Drying Modality Affect Interstitial Assemblages of Alluvial Wetlands

Abstract

Long-term changes (river incision, climate changes and groundwater overuse) induce increasing groundwater depletion and drought frequency in floodplains. To evaluate the effect of drought on functioning of wetlands and their interstitial zones, we studied the interstitial communities at seven stations located in five former channels of the Rhone River. Change in interstitial assemblages was assessed over a 32-year period in one wetland and over the entire floodplain between 1987 and 2010, at depths of 50 and 100 cm within the sediment. Over the 32-year period, we highlighted some links between environmental changes and variations in interstitial assemblages: a decrease in abundance and diversity of the hypogean fauna and a reverse pattern for Ephemeroptera and Cladocera. At the floodplain scale, several changes in habitat characteristics were observed and two drying modalities were identified: the formation of isolated pools and the complete disappearance of surface water. In isolated pools, the low turnover of surface water promoted a decrease in water quality, an increase in the abundance of benthic invertebrates in the interstitial habitat and a decrease in the abundance of hypogean invertebrates. In completely dried-out sites, the abundance and diversity of benthic taxa decreased, while the hypogean fauna remained dominant. These results demonstrate that interstitial assemblages are affected by long-term changes (river incision and climate changes). The local geomorphology and the resulting drying modalities control the responses of interstitial assemblages.