Incorporating the hyporheic zone within the river discontinuum: Longitudinal patterns of subsurface copepod assemblages in an Alpine stream

At coarse spatial scales, Alpine streams are patchy discontinuum environments from headwaters to mouth. This condition may have profound implications on the structure and function of the hyporheic ecotone. This study was aimed at assessing whether meiofaunal copepod assemblages displayed a longitudinal zonation corresponding to discontinuities along an Alpine stream profile determined by changes of geology and alternating canyon and floodplain segments. Moreover, the study aimed at testing, at a regional scale, the potential role of copepods as describers of surface-subsurface hydrological exchanges in the hyporheic zone. The results indicated that copepod assemblage compositions changed according to geomorphological and physicochemical stream longitudinal discontinuities. The headwater upwelling sector of the stream predominantly harboured stygobiotic species; the intermediate sector, where stream water is in hydrodynamic equilibrium with ground water, was characterised by the highest species richness and abundances both of stygobiotic and non-stygobiotic species; the downstream alluvial fan, where downwelling occurred, was defined by high non-stygobiotic species richness along with low occurrence of stygobionts. The different distribution of stygobiotic and non-stygobiotic species in gaining and losing sectors of this Alpine stream suggested the role of copepods as describers of the hydrological flowpaths in the hyporheic zone at the catchment scale.