Low‐Head Dam Removal Restores Biofilm Structure but Not Function in a Temperate Stream

ABSTRACT Barrier removal is a common stream restoration practice aimed at restoring longitudinal connectivity, yet its effects on biofilm structure and function, through alteration of near‐bed hydrodynamics, remain unclear. Using a space‐for‐time substitution approach, we assessed how the presence and removal of a low‐head dam affect biofilm structure and function. We quantified near‐bed hydrodynamics and biofilm structure and function across three reaches in a temperate stream: one impacted by a low‐head dam, one in reference condition, and one restored where a low‐head dam was removed over a decade ago. In each reach, we quantified near‐bed hydrodynamics, and biofilm structural (microbial α ‐diversity, biomass, Chlorophyll a , bacterial abundance) and functional parameters (nitrate (N‐NO 3 − ) and dissolved organic carbon (DOC) uptake), along with microbial community composition. We found that the low‐head dam altered near‐bed hydrodynamics, as well as biofilm structure and function. Restoration successfully reestablished near‐bed hydrodynamics similar to those observed at the reference site, which coincided with the recovery of impaired biofilm structural parameters. However, biofilm DOC uptake remained significantly lower in the restored reach compared to the control, indicating a persistent impairment despite restoration. Presence–absence patterns of specific taxa explained a small (15%), but consistent, fraction of the variance in DOC uptake, suggesting that the occurrence of particular microbial groups may be linked to the incomplete DOC uptake recovery. Our results suggest that dam‐induced alterations in near‐bed hydrodynamics largely explain the structural changes observed in biofilms. Restoring near‐bed hydrodynamics supports the recovery of biofilm structure, but functional recovery remains incomplete. Therefore, hydromorphological restoration alone, while necessary, is unlikely to fully restore ecosystem functioning. Our study highlights the need to integrate biological and biogeochemical targets when assessing restoration success.