Human activities (e.g., agriculture, urbanization) have largely increased inputs of fine sediments in aquatic environments. Streambed clogging induced by fine sediment deposition and infiltration into streambed sediments is today considered as a major environmental concern throughout the world. The hyporheic zone (HZ), defined as the interface between surface water and groundwater in stream and river beds, experiences intense hydrological exchanges, supports significant biogeochemical processes and harbours rich and diverse microbial communities. Nevertheless, the ecological effects of streambed clogging have rarely been addressed considering the HZ of streams. The main objective of this study was to determine the influence of sediment clogging on hyporheic microbial structure and functions of three French rivers (the Usses, Drôme and Isere rivers). In each river, microbial abundances and activities were studied at three depths (10, 30 and 50 cm) of the hyporheic sediment in one unclogged site (with a high porosity) and one clogged site (with a low porosity).

Results/Conclusions

Previous studies predicted that sediment clogging due to fine particle deposition could induce an intense bacterial activity, lowering oxygen availability in the hyporheic zone and so, decreasing aerobic microbial processes (nitrification, respiration) and enhancing anaerobic processes such as denitrification, fermentation, and methanogenesis. Our results showed that the sediment clogging induced contrasting effects on microbial processes in the different rivers. Depending on the studied river, global increases (Usses river) or decreases (Drôme and Isere rivers) of both aerobic and anaerobic processes were detected in the clogged sites compared to the unclogged ones. These results suggested that the microbial changes due to the sediment clogging would be mainly mediated by water residence time within hyporheic sediments. A single model predicting the effect of clogging on hyporheic microbial processes cannot be applied generally for all rivers because the flow rates between surface and interstitial waters were probably not linearly correlated with the clogging intensity. As a consequence, further studies are needed to assess the influence of heterogeneous clogging on water dynamics and exchanges between surface and interstitial waters that control microbial processes in river sediments.