Effects of management works on the interstitial fauna of floodplain aquatic systems (River Rhone, France)

Long-term changes in composition, structure and biodiversity (i.e. taxonomi c richness, diversity index, species traits and habitat-affinity) of inters titial assemblages were studied in two floodplain systems: a restored backw ater and an artificial drainage canal. Before restoration, the backwater, a ffected by both terrestrialisation and eutrophication, was weakly populated by a low diversified fauna dominated by walkers, macrofauna, detritivores, and stygoxenes (i.e. taxa that occur incidentally in ground waters) that r eproduce biparentally and lack parental care. This backwater displayed an u pstream-downstream gradient in response to restoration works. Upstream, the dredging of fine organic sediments favoured inputs of nutrient-poor ground water and exchanges between groundwater and surface water that induced an i ncrease in taxonomic richness (in both herbivores and stygoxenes). Downstre am the deposition of fine sediment that was suspended in the water column b y restoration work enhanced colmation that induced a decrease in herbivore and swimming taxa, and an increase in mesofaunal taxa, whilst phreatobites (i.e. taxa specialized to interstitial life) remained absent from the syste m. The drainage canal that was artificially hollowed-out to lower the surro unding water table, harbors mixed assemblages of epigean (i.e. taxa of surf ace-water habitats) and hypogean (i.e. taxa of groundwater habitats) taxa. The upstream part, which is weakly influenced by surface waters, was coloni zed by phreatobites as oligotrophic conditions increased. The intermediate part, which is fed by surface water and where mesotrophic conditions occurr ed as habitats progressively matured and diversified, showed diversificatio n of its fauna. The downstream part of the drainage canal displayed the rev erse dynamic - this suggests a reduction in groundwater supply due to the c logging of sediment interstices fine sediments, the deposition of which is linked to the Rossillon backwater restoration works.