Spatial distribution patterns of Crustacea in the floodplain aquifer of an alluvial river

Abstract

Interstitial crustaceans were collected from a grid of shallow sampling wells penetrating the alluvial floodplain aquifer (ca. 10 m × 5 km × 10 km) of the Flathead River, Montana, USA. Eighteen taxa were identified, which collectively encompassed a range of hypogean-epigean affinities. The subterranean amphipod Stygobromus spp., the most common crustacean, occurred in all wells but was rare in the channel well. When well data were pooled into ‘habitat types’ (channel, bank, near-, central-, and far-floodplain), distinct faunal patterns were apparent. Crustaceans constituted an increasing percentage of the total interstitial fauna from the channel to the near-floodplain, then maintained similar relative abundance levels with increasing distance from the river. Stygobionts attained maximum values at near- and central-floodplain habitats where copepods and ostracods dropped to the lowest levels. Distribution and abundance patterns of Crustacea at the floodplain scale are structured by hydrogeologic and geomorphic processes reflected only in part by distance from the river channel. The flood plain appears to contain a latticework of alluvial-filled paleochannels of high hydraulic conductivity that induce spatial discontinuities within the aquifer and that may play an important role in determining crustacean distribution patterns.