Impact of hydropower development on biofilm prokaryotic communities and organic matter breakdown in Norwegian rivers.

Hydropower development in river systems has a major impact on river systems, changing the natural flow regime and hindering the connectivity between upstream and downstream reaches. Flow alteration can have effects on water quality, sediment transport, nutrient cycling, and therefore on river biota. Biofilms act as interface layers between water and sediment, crucial for the biogeochemical processes in river ecosystems. Although their important role, studies addressing the effects of hydropower on the community structure and functioning of these microbial communities are still scarce. Here we report results on the diversity and structure of biofilm prokaryotic communities and the efficiency in organic matter breakdown for ten rivers impacted by different kinds of hydropower developments. Comparing free flowing river reaches to sites impacted by dams for electric power production, we were able to detect significant differences in the decomposition of organic matter, with impacted sites being more effective. The prokaryotic diversity of epilithic biofilm did not show any significant change and overall the species composition was similar for the impacted and unimpacted river reaches. Our study suggests that river fragmentation due to hydroelectric powerplants may have a significant effect on functional aspects of the ecosystems, altering the organic matter degradation processes, even though microbial diversity seems to be unaffected. Altered flow regimes can have negative effects on nutrient cycling and ecosystem processes, it is therefore important to further investigate the impact on functions mediated by microbial communities.