Distinct changes in riparian sediment microbial communities with depth and time since dam removal

Microbes drive biogeochemical changes in ecosystems, including carbon (C) and nitrogen (N) cycling. Dam construction has altered riparian ecosystems worldwide, yet we know little about microbial community composition in riparian sediments and how it changes following dam removal and sediment/soil drainage. Here, we evaluate how riparian microbial communities change with increasing depth in the sediment profile for existing dams and over time following dam removal/breach and assess how various physico-chemical sediment properties influence microbial community composition. We studied microbial community structure for 12 riparian sites over a chronosequence of 0-234 years since dam breach. Sediment was collected every 0.3 m to a depth of 4 m. Aerobic taxa involved with N cycling (e.g., Nitrospirota) were dominant in surficial sediments, and increased in deeper sediments as time since dam breach increased. Anaerobic taxa implicated in C cycling (e.g., Bathyarchaeia, Anaerolineaceae) and iron reduction (e.g., Sva0485) were dominant in deeper, anoxic sediments, but declined the fastest post dam breach. These microbial trends provide insights into how riparian biogeochemical functions are impacted by dam inundation and the recovery and restoration of these ecosystems following dam removals.