In Arctic catchments, bacterioplankton are dispersed through soils and streams, both of which freeze and thaw/flow in phase, seasonally. To characterize this dispersal and its potential impact on biogeochemistry, we collected bacterioplankton and measured stream physicochemistry during snowmelt and after vegetation senescence across multiple stream orders in alpine, tundra, and tundra-dominated-by-lakes catchments. In all catchments, differences in community composition were associated with seasonal thaw, then attachment status (i.e. free floating or sediment associated), and then stream order. Bacterioplankton taxonomic diversity and richness were elevated in sediment-associated fractions and in higher-order reaches during snowmelt. Families Chthonomonadaceae, Pyrinomonadaceae, and Xiphinematobacteraceae were abundantly different across seasons, while Flavobacteriaceae and Microscillaceae were abundantly different between free-floating and sediment-associated fractions. Physicochemical data suggested there was high iron (Fe+ ) production (alpine catchment); Fe+ production and chloride (Cl- ) removal (tundra catchment); and phosphorus (SRP) removal and ammonium (NH4 + ) production (lake catchment). In tundra landscapes, these 'hot spots' of Fe+ production and Cl- removal accompanied shifts in species richness, while SRP promoted the antecedent community. Our findings suggest that freshet increases bacterial dispersal from headwater catchments to receiving catchments, where bacterioplankton-mineral relations stabilized communities in free-flowing reaches, but bacterioplankton-nutrient relations stabilized those punctuated by lakes.
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