Geomorphic influence on intraspecific genetic differentiation and diversity along hyporheic corridors

Abstract : 1. The hyporheic zone of rivers potentially acts as a dispersal corridor for ground- water organisms because it provides a spatially continuous interstitial habitat between isolated aquifers. Yet, the degree to which it can facilitate the move- ment of organisms has been hypothesized to vary in response to change in sedi- ment regime, which determines channel morphology. 2. In this study, we used microsatellite markers to test for a relationship between the genetic structure and diversity of the minute interstitial isopod Proasellus walteri and channel morphology along three nearby hyporheic corridors differing widely in their sediment regime. We predicted that genetic diversity would decrease and genetic structuring would increase as sediment supply-limited channels would become prominent features in the river corridor. The reason is that such channels have fewer and less suitable sedimentary habitats for migra- tion because they lack large depositional bedforms such as gravel bars. 3. Using genotypic data from seven microsatellite loci for a total of 713 individuals distributed among 25 demes, we found that demes had on average more alleles and were less differentiated in the river showing the most extensive alluvial depos- its and shortest length of sediment supply-limited channels. Population clusters were also of greater size, reaching up to 30 km in length. The longitudinal pattern of genetic differentiation in this sediment-rich river was best explained by hydro- logic distance and the longitudinal pattern of allelic richness was bell-shaped, as expected under a stepping-stone model with symmetrical migration. 4. The length of sediment supply-channels was more important than hydrologic dis- tance in explaining the longitudinal distribution of genetic differentiation in the two other corridors facing a sediment shortage. Allelic richness decreased mono- tonically upstream in the most sediment-poor river. This correlates with the expan- sion further downstream of sediment supply-limited channels in this river, which is likely to decrease animal movement and hence gene flow among demes. 5. Thisstudyprovidesthefirstevidencethatthedegreetowhichthehyporheiczone facilitates the movement of groundwater organisms varies greatly among rivers of contrasted geomorphology. Extending the application of riverscape genetics across a range of interstitial taxa and geomorphic settings holds much promise for assessing the contribution of the hyporheic zone to the dispersal of groundwater organisms.