Influence of groundwater upwelling on the distribution of the hyporheos in a headwater river flood plain

We examined the influence of groundwater upwelling on the distribution of the hyporheic fauna in a 2.6-km long glacial flood plain, the downstream part of which behaved as a major groundwater upwelling zone. Hyporheic water and invertebrates were collected during the four different phases of the expansion-contraction cycle at a depth of 30cm in the bed sediment of 28 channels. Multivariate methods and analysis of variance were used to test for differences in the concentration of nutrients and composition of the hyporheos between the upstream and downstream parts of the flood plain. The large scale groundwater upwelling flow pattern maintained permanent flow conditions in the lower flood plain but had no detectable influence on temperature and nutrient concentrations of shallow hyporheic water. In contrast, the density and taxonomic richness of hyporheic assemblages were distinctly higher in the upwelling zone, essentially because of an increase in the number and density of taxa belonging to the permanent hyporheos and true groundwater fauna. The longitudinal pattern of hyporheic assemblages remained remarkably stable over the four different phases of the expansion-contraction cycle. We concluded that the strength of hydrological connections between ground water and surface water had a major influence on hyporheic biodiversity by controlling not only the degree of flow permanency but also the recolonization pathways of organisms inhabiting streambed sediments.