Prokaryotic community structure under different flow regimes. Birmingham experiment

River ecosystems worldwide are affected by dramatic alterations, most of them caused by human impacts. Modifications to the natural flow regime are among the major threats to the ecological integrity of these environments and, future climate change scenarios are expected to worsen the negative effects. Research efforts have so far focused mainly on the consequences for fish, macroinvertebrates and riparian vegetation, while microbial communities have been poorly addressed. Here we report the results of a 100 days mesocosm study, where we analysed the relationships between different flow scenarios and the effects on the biofilm prokaryotic community structure and functioning over time. By using fine and coarse mesh bags with standardized cloth stripes as substrates for biofilm growth, we analysed the structure (16S rRNA gene metabarcoding) and function (organic matter breakdown) across four different flow regimes and in two habitats (pool and riffle). Drought treatments showed the greatest impact on both, biofilm community composition and organic matter breakdown, which was always significantly higher compared to the other treatments. Overall the species turnover (beta diversity) showed significant changes related to the flow scenarios, while no significant effects were observed on the alpha diversity. Linear mixed effects models on the organic matter breakdown showed that sampling times and treatments were the variables with highest influence. These findings highlight the importance of flow regimes on microbial community structure and function in river ecosystems with potential implications for the ecological integrity of key ecosystem processes such as Carbon and Nitrogen cycling and possible bottom-up effects on the riverine food webs.