Spatio-temporal dynamics of bacterial community composition in a Western European watershed, the Meuse River watershed.

This study aimed to identify factors influencing bacterial diversity in the Meuse River watershed by analyzing 42 locations sampled in spring and summer 2019, combined with biweekly sampling of one mid-stream location for a year. Bacterial community composition (BCC) was assessed in the small (SF; <5 µm) and large fractions (LF; ≥5 µm,), alongside physico-chemical parameters. LF consistently exhibited greater alpha diversity than SF. During the spatial campaigns, alpha diversity increased downstream in spring with high discharge, and BCC differed significantly between headwaters and the main river. Along this axis, several genera, Flavobacterium, Limnohabitans, and Aquirufa stood out as indicators of good water quality. Rhodoferax, another taxon indicative of good water quality, prevailed in the headwaters and during winter. In contrast, two cyanobacteria genera indicators of poor river quality, Microcystis PCC 7914 and Cyanobium PCC 6307, peaked in summer. BCC in spring and summer temporal samples aligned with spatial ones, while winter and autumn samples had distinct BCC. Finally, season, temperature, and distance from river mouth were the main driving parameters of beta diversity, outweighing the effect of fraction size on the BCC. These findings reinforce the notion that local conditions exert significant influence on bacterial communities in rivers.