Evidences of linkages between biofilm communities and ecosystem functioning at stream-reach scale derived from stable isotopes

We examined the relationship between nitrification and biofilm community composition at reach scale in a stream receiving inputs from a wastewater treatment plant (WWTP). We collected water and biofilm samples along the reach for dissolved inorganic nitrogen (N) concentration, ¹⁵N natural abundances and bacterial characterization. Sampling was done twice, in late winter and late summer 2008. Longitudinal patterns of stream N concentrations together with those of ¹⁵N natural abundances below WWTP inputs indicated that these sites are hotspots for NH₄-N assimilation and nitrification, supporting previous studies. In addition, these two processes are spatially segregated at microhabitat scale, as indicated by a significant difference in the ¹⁵N signature of biofilm communities developed in the light (signatures close to those of ¹⁵N-NH₄) and dark (signatures depleted compared to ¹⁵N-NH₄) sides of cobbles. These results suggest that nitrification is mostly driven by biofilms coating the “dark side” of cobbles. Fluorescence in situ hybridization confirmed nitrifiers in these micro-habitats. These nitrifiers were similar to those from the WWTP, indicating the successful colonizing capacity of WWTP-derived bacteria in the recipient stream even 1 km downstream of the source. Collectively, these results link epilithic biofilms and N transformation at the reach scale in a N-rich stream.