First survey of interstitial fauna in New Caledonian rivers: influence of geological and geomorphological characteristics

Despite zoogeographical implications and geological anomalies, the intersti tial fauna of New Caledonian rivers remains unstudied. During this first su rvey, 15 different sized rivers varying in sediment grain size and geologic al substratum were sampled for both benthic (Surber sampler) and interstiti al (Karaman-Chappuis pits) invertebrates during summer 1998-99. A total of 34 taxa was collected during the study, of which 22 were present in the sed iment interstices. Interstitial assemblages were dominated by the Oligochae ta (present in 80% of the samples), the Chironomidae (in 73%), the Ceratopo gonidae (in 73%), the Coleoptera Berosini (in 47%), the Turbellaria (in 47% ), the OstracodaVestalenula sp. (in 40%) and the Copepoda Cyclopoida (in 40 %). New amphipods close to Bogidiellidae were found in two stations. Some o rganisms, frequent and abundant in the benthic layer, were rare or absent i n the sediment interstices (e.g. Decapoda Atyidae, Ephemeroptera Leptophleb iidae, Trichoptera Hydropsychidae and Leptoceridae, Diptera Simulidae). Riv er size and distance between the station and the ocean were unassociated wi th abundance, taxonomic richness or composition of interstitial assemblages . In contrast, both abundance and taxonomic richness were significantly low er when sediment grain size decreased (probably because of low exchange wit h surface assemblages) and when the percentage of peridotitic substratum in the catchment increased (probably because of its high nickel and chromium contents). These preliminary results suggest that interstitial fauna can be used for ecological survey and river management even in tropical regions.