Impact of hydrology on free-living and particle-associated microorganisms in a river floodplain system (Danube, Austria)

1. A floodplain segment of the Danube River downstream of Vienna was studied during the hydrologically most dynamic phase (spring–summer) to evaluate the significance of connection between the main channel and the floodplain segment for particle abundance and quality as well as for bacterial and viral parameters, both free‐living and attached to particles. 2. Hydrological connectivity between the main channel and its floodplain segment (expressed as water age) influenced particle abundance and quality. Polysaccharide‐containing particles [Alcian positive‐stained particles (ASP)] and protein‐containing particles [Coomassie positive‐stained particles (CSP)] each contributed a substantial fraction to total suspended solids and were both positively related to water age. ASP were about twice as abundant as CSP. 3. Water age influenced bacterial and viral abundance and the bacterioplankton productivity in the surrounding water. Free‐living bacterial abundance and their bacterial secondary production (BSP) increased continuously with water age, best described by a linear regression. Water age also significantly impacted BSP and per cell BSP of bacteria attached to particles. The abundance of attached bacteria and viruses was not influenced by water age. 4. Bacteria and viruses on particles were influenced by particle quality. Their abundance on particles was closely related to particle size. Particle‐attached bacteria accounted on average for 30.34% (± 3.09) of the total bacterial abundance. A variable and occasionally significant proportion of viruses, between 0.43% and 35.06%, were associated with particulate material. 5. Bacteria attached to particles were significantly more productive than their free‐living counterparts. Their per‐cell activity was on average 8.6 times higher than that of the free‐living fraction. 6. Hydrological connectivity between the Danube River and its floodplain is crucial not only for the exchange of water, sediment and nutrients, but also for microbiota, thus influencing microbial life, distribution and activity.