Temporal Community Assembly of Bacteria and Protists in Different Habitats of a Mountain Lake

Whether microbial community assembly is primarily deterministic (selection-driven) or stochastic (dispersal-driven) remains a central debate in microbial ecology. We investigated community assembly of bacteria and protists and their driving ecological processes in mountain Lake Tovel using a 5-year 16S and 18S rRNA metabarcoding dataset. The frequencies of assembly mechanisms and their processes depend on the considered temporal scale. We assessed the influence of temporal scale (years, seasons, temporal distances from 1 to 48 months across all years) and year-pooling (a common strategy in the assessment of assembly patterns) on the inference of community assembly in the upper euphotic (0-3 m), lower euphotic (3-25 m) and deep hypolimnetic (30-35 m) layers. Focusing on single years, bacteria showed more often different assembly mechanisms (both deterministic and stochastic) across layers and years than protists. Year-pooling hid the inter-year differences and preserved the signal of the most frequent single-year process. A lagged effect of water temperature on bacterial community assembly was found, while an immediate effect on protist assembly. Bacterial and protists' niche width was generally independent of year-pooling and increased with depth. Our study showed that different temporal scales and depth layers should be considered to gain a comprehensive overview on community assembly.