Influence of hyporheic zone characteristics on the structure and activity of microbial assemblages

Summary The hyporheic zone is a pivotal environment for biogeochemical processes in riverine ecosystems. However, the responses of hyporheic microbial assemblage structure and activity to environmental variables remain poorly understood. The objective of this study was to determine the influence of hyporheic zone characteristics on microbial assemblages in three French rivers (Usses, Isère and Drôme). For each river, microbial structure (assemblage composition and abundance) and activity were measured at three sites with contrasting environmental characteristics (e.g. effective streambed porosity, oxygen and nutrient concentrations). Firstly, we expected that the structure and activity of the hyporheic microbial assemblage would be significantly affected by the environmental characteristics of the hyporheic zone. We also expected a strong relationship between the structure and activity of hyporheic microbial communities in the Usses River, due to greater spatial heterogeneity (i.e. greater porosity differences between the studied sites) compared with the two other rivers. Our results showed that porosity reduction, greatest in the Usses River, affected the exchanges of flows and energy through the hyporheic zone and, consequently, led to a distinct microbial assemblage composition in comparison with the two other rivers. Significant relationships between bacterial assemblage structure and activity were detected for the Usses and Drôme rivers, due to the greater spatial variability in sediment and physicochemical variables observed in these two rivers than in the Isère. Hyporheic zone characteristics (sediment permeability, water chemistry, particulate organic matter) appear to be the key factors controlling microbial structure and activity in riverine environments. As the hyporheic zone is essential for biogeochemical processes, including those attenuating pollution, a better quantification of the influences of environmental factors controlling microbial activity is needed to understand and predict ecosystem processes in the hyporheic zone.