Hydrological regime of a continental river system predicts bacterial macroecological patterns

Modelling bacterial dynamics in large river systems is crucial for predicting continental-scale ecosystem functioning under anthropogenic pressures. Although the River Continuum and Metacommunity concepts have provided theoretical frameworks, quantitative parameters necessary for microbial macroecological models remain scarce. Here, we present results from two whole-river surveys, conducted six years apart along 2600 km of the Danube River. Using bacterial secondary production, cell counts, and 16S rRNA gene amplicon sequencing, we quantified carbon, cell, phylotype, and diversity turnover along the river. Carbon incorporation per cell declined with water travel time by 6,000 - 21,000 atoms per hour. Bacterial cells multiplied every eight days, resulting in four to six doublings during downstream transport. Growth responses at the level of individual phylotypes differed up to a hundredfold from these bulk community estimates. Bacterial diversity dynamics were dominated by phylotype turnover rather than phylotype loss. Turnover ranged 0.92 to 0.96 across the river, indicating an almost complete replacement of phylotypes with 2-11% of headwater-associated ASVs persisting under base-flow conditions. Richness declined gradually downstream at a rate of approximately 0.13 ASVs per hour. Variations in bacterial secondary production, cell abundance, and observed ASVs were best explained by models combining hydrological and water quality parameters, whereas beta diversity followed a gradual development primarily structured by water travel time. Together, these results identify water travel time as the key integrative parameter governing microbial macroecological dynamics along large rivers, with environmental conditions fine-tuning local responses. These models can help predict changes in microbial diversity and functioning under anthropogenic alterations.