Soil Microbial Inoculation during Flood Events Shapes Stream Microbial Communities and Diversity

Flood events are now recognized as potentially important occasions for the transfer of soil microbes to stream ecosystems. In this study, the influence of soil inoculation during high flow events on stream bacterial community composition and diversity was explored in six pre-alpine first order streams and the larger Oberer Seebach. During one year, variations of community composition in soil water, stream water and in benthic biofilms at different flow conditions (low to intermediate flows versus high flow) were compared. Bacterial diversity was lowest in biofilms, followed by soils and highest in headwater streams and the Oberer Seebach. In headwater streams, bacterial diversity was significantly higher during high flow, as compared to low flow. Approximately 70% of the bacterial operational taxonomic units from streams and stream biofilms were the same as in soil water, while in the latter one third of the OTUs were specific to high flow conditions. These soil high-flow OTUs were also found in streams and biofilms at other times of the year. These results demonstrate the relevance of floods in generating short and reoccurring inoculation events for flowing waters. Moreover, they show that soil microbial inoculation during high flow enhances microbial diversity and shapes fluvial BCC even during low flow. Hence, soil microbial inoculation during floods could act as a previously overlooked driver of microbial diversity in headwater streams.