Effects of streambed morphology and biofilm growth on the transient storage of solutes.

Microbial biofilms are the prime site of nutrient and contaminant removal in streams. It is therefore essential to understand how biofilms affect hydrodynamic exchange, solute transport, and retention in systems where geomorphology end induced hydrodynamics shape their growth and structure. We experimented with large-scale streamside flumes with streambed landscapes constructed from graded bedforms of constant height and wavelength. Each flume had a different bedform height and was covered with a layer of gravel as substratum for benthic microbial biofilms. Biofilms developed different biomass and physical structures in response to the hydrodynamic conditions induced by the streambed morphology. Step injections of conservative tracers were performed at different biofilm growth stages. The experimental breakthrough curves were analyzed with the STIR model, using a residence time approach to characterize the retention effects associated with biofilms. The retained mass of the solute increased with biofilm biomass and the biofilm-associated retention was furthermore related to bedform height. We tentatively relate this behavior to biofilm structural differentiation induced by bed morphology, which highlights the strong linkage between geomorphology, hydrodynamics, and biofilms in natural streams and provide important clues for stream restoration.