Past dispersal in a lake bacterial communtiy
Lake bacterial communities are predominantly shaped by environmental selection and dispersal from the surrounding catchment. Strong seasonal dispersal events (e.g., high discharge from the inlet) can lead to a high cell load from the tributary, often carrying bacterial cells originating from the surrounding terrestrial environment. Upon arrival in the downstream lake, the bacteria can get filtered by the lake environment, resulting in cell death or dormancy. Alternatively, inactive cells that reach suitable substrate and habitat conditions can shift in activity and actively grow in the lake, thus, contributing to the lake bacterial community composition (BCC). If bacterial seeds shift in activity at a later time point, the effect of dispersal on the lake community might be delayed but such legacy effects have been little studied. In our study, we investigated the potential importance of a past strong dispersal event from the main inlet (“spring flood event”) to a natural lake bacterial community using SourceTracker analysis. Further, we aimed to identify contributing ASVs to the delayed effects and inspected their relative abundance and activity along the aquatic continuum using RNA:DNA ratio captured by 16S rRNA amplicon sequencing.  We found that past spring flood dispersal mattered for the lake BCC for up to four weeks, however, past dispersal was of minor importance compared to present-day dispersal from the lake hypolimnion and the inlet. The signal of past dispersal in the lake community mostly resulted from the dispersal of rare and inactive bacterial cells.