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Abstract Gas transfer velocity () controls gas fluxes between aquatic ecosystems and the atmosphere. In streams, is controlled by turbulence and, thus, local hydrology and geomorphology. Resultantly, variability in can be large and modeling from physical parameters can have large uncertainty. Here, we leverage a large dataset of estimates derived from tracer‐gas experiments in 22 US streams across a range of discharges. Our analysis shows that was highly variable both spatially across and temporally within streams, with estimates of spanning three orders of magnitude. Overall, scaled with discharge in steep streams due to relatively high stream power, but not in low‐slope streams, where stream power was relatively low even at high flows. Understanding how responds to stream discharge in a wide variety of streams is key to creating temporally and spatially resolved estimates of biogeochemical processes in streams. 

Abstract There is growing evidence that the composition of river microbial communities gradually transitions from terrestrial taxa in headwaters to unique planktonic and biofilm taxa downstream. Yet, little is known about fundamental controls on this community transition across scales in river networks. We hypothesized that community composition is controlled by flow‐weighted travel time of water, in combination with temperature and dissolved organic matter (DOM), via similar mechanisms postulated in the Pulse‐Shunt Concept for DOM. Bacterioplankton and biofilm samples were collected at least quarterly for 2 yr at 30 sites throughout the Connecticut River watershed. Among hydrologic variables, travel time was a better predictor of both bacterioplankton and biofilm community structure than watershed area, dendritic distance, or discharge. Among all variables, both bacterioplankton and biofilm composition correlated with travel time, temperature, and DOM composition. Bacterioplankton beta‐diversity was highest at shorter travel times (< 1 d) and decreased with increasing travel time, showing progressive homogenization as water flows downstream. Bacterioplankton and biofilm communities were similar at short travel times, but diverged as travel time increased. Bacterioplankton composition at downstream sites more closely resembled headwater communities when temperatures were cooler and travel times shorter. These findings suggest that the pace and trajectory of riverine bacterioplankton community succession may be controlled by temperature‐regulated growth rate and time for communities to grow and change. Moreover, bacterioplankton, and to a lesser extent biofilm, may experience the same hydrologic forcing hypothesized in the Pulse‐Shunt Concept for DOM, suggesting that hydrology controls the dispersal of microbial communities in river networks.