Intensified organic carbon dynamics in the ground water of a restored riparian zone

1. River restoration projects usually aim at improving the physical habitat for aquatic organisms. The extent to which biogeochemical processes and microbial activities are intensified in restored river reaches remains uncertain. 2. Here, we investigated the relationships between the distribution and composition of organic carbon (OC), bacterial secondary carbon production and extracellular enzymatic activity (EEA) in the ground water below a restored riparian section of the River Thur, Switzerland, relative to a channelised section. The spatiotemporal variability in the stable C isotopic ratio, dissolved OC polydispersity (the distribution of molecular mass in a mixture of molecules) as well as bacterial abundance, EEA and secondary production were investigated in different process zones. 3. At high river discharge, humic as well as low molecular weight amphiphilic substances infiltrated into the subsurface in a zone dominated by the pioneer plants Salix viminalis (willow bush). Concurrently, bacterial abundance, EEA and secondary carbon production increased at this location. 4. The willow plants leached bioavailable substrates into the ground water when the water table was high. The flood‐driven soil–groundwater coupling stimulated EEA and bacterial secondary production of the suspended groundwater bacterial community. 5. Establishing riparian habitat diversity adds hot spots of OC inputs during flood events, potentially providing valuable ecosystem services (e.g. degradation of organic pollutants) that accompany.