A comparison of longitudinal patterns in hyporheic and benthic oligochaete assemblages in a glacial river

This paper examines the longitudinal pattern of oligochaetes in the hyporheic and benthic layers of a glacial river (Val Roseg, Switzerland). We hypothesized that the spatial distribution of oligochaete communities would reflect changes in surface water/groundwater interactions along the river continuum. From June to November 1997 (4 dates), three hyporheic and three benthic samples were collected at 11 and 9 sites, respectively, located over a distance of 11 km from the glacier terminus. A two-ended member mixing model based on concentrations of sodium was used to determine changes in the relative proportions of glacial water and groundwater along the river. The longitudinal increase in the diversity and abundance of hyporheic assemblages of oligochaetes was closely linked to the convergence of groundwater and surface glacial water. The differential distribution of hyporheic and benthic assemblages suggested that the hyporheic corridor acted as the main upstream migration pathway for oligochaetes. In this glacial river, the hyporheic zone appeared as a source area from which benthic habitats presenting suitable environmental conditions could be colonized.