Community assembly at different temporal scales

The assessment of assembly processes (AP) is important for management and conservation in the light of climate change and anthropogenic pressures. We investigated AP at different temporal scales for bacteria and protists in the upper and intermediate layer and the hypolimnion of mountain Lake Tovel for five years based on high-throughput sequencing. The assessment of AP is based on pair-wise comparisons of communities and their expression as relative occurrence with respect to the total number of comparisons. We considered the following different temporal scales, that constrained the total number of comparisons: (i) across all years; (ii) within years; (iii) for seasons across years; (iv) for seasons within years; (v) for monthly distances across all years; (vi) for monthly distances within years. The similarity of AP between bacteria and protists depended on the temporal scale and layers considered while general statements on the dominance structure and frequency distribution of AP were generally independent of the temporal scale considered. This was reassuring because it showed that other studies merging data across temporal scales were not fully misled. However, the considerations of monthly distances and the consideration of single years at yearly and seasonal scale revealed temporal exceptions, that otherwise remained unnoticed. Especially the cyclical pattern of AP at monthly distances showed that processes are quite dynamic and linked to environmental parameters. Furthermore, AP of the hypolimnion were different from those of the upper two layers. To gain a comprehensive overview on AP, different temporal scales and depth layers should be considered.