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Abstract Freshwater wetlands process large amounts of nutrients originating from agricultural fields. Yet, these systems also have the potential to produce substantial amounts of nitrous oxide (N₂O) and methane (CH₄), both potent greenhouse gasses (GHGs). Agricultural land use alters delivery of nutrients and carbon (C) to downstream wetlands, and changing climate is altering hydroperiods. These drivers modulate wetland microbial processes responsible for GHG production including denitrification and methanogenesis. Studies have correlated GHGs to C quantity and nutrients independently; fewer studies identify how nutrients and C composition interact to modulate GHG concentrations in wetlands. In wetlands located in Indiana, USA, we studied how CH₄, N₂O, and carbon dioxide (CO₂) correlated to C quantity and composition, nutrient concentrations, size, hydrology, and surrounding agricultural land use. CH₄production was correlated to dissolved organic carbon (DOC) concentrations and composition using UV‐Vis spectroscopy. CH₄concentrations were positively correlated to spectral slope from 275 to 295 nm, an indicator of autochthonous primary production, and negatively correlated to humification index. N₂O concentrations positively correlated to total dissolved nitrogen and humification index (HIX). CH₄concentrations were highest in the large wetland with negligible canopy cover, dense macrophytes and algae, and high concentrations of autochthonous‐like DOC. Thus, we suspect phototrophic methanogenesis is an important driver of CH₄variation across systems. Concentrations of N₂O were highest in the agricultural wetland, likely driven by higher NO₃⁻concentrations. Our findings suggest agricultural nutrients strongly shift greenhouse gas production profiles but do not necessarily increase global warming potential of GHGs released by wetlands. 

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.