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Streams and rivers are major sources of greenhouse gases (GHGs) to the atmosphere, as carbon and nitrogen are converted and outgassed during transport. Although our understanding of drivers of individual GHG fluxes has improved with numerous site‐specific studies and global‐scale compilations, our ability to parse out interrelated physical and biogeochemical drivers of gas concentrations is limited by a lack of consistently collected, temporally continuous samples of GHGs and their associated drivers. We present a first analysis of such a dataset collected by the National Ecological Observatory Network across 27 streams and rivers across ecoclimatic domains of the United States. Average concentrations of CO₂ranged from 36.9 ± 0.88 to 404 ± 33 μmol L⁻¹, CH₄from 0.003 ± 0.0003 to 4.99 ± 0.72 μmol L⁻¹, and N₂O from 0.015 to 0.04 μmol L⁻¹and spanned ranges of previous global compilations. Both CO₂and CH₄were strongly affected by physical drivers including mean air temperature and stream slope, as well as by dissolved oxygen and total nitrogen concentrations. N₂O was exclusively correlated with total nitrogen concentrations. Results suggested that potential for gas exchange dominated patterns in gas concentrations at the site level, but contributions of in‐stream aerobic and anaerobic metabolism, and groundwater also likely varied across sites. The highest gas concentrations as well as highest variability occurred in low‐gradient, warmer, and nonperennial systems. These results are a first step in providing unprecedented, continuous estimates of GHG flux constrained by temporally variable physical and biogeochemical drivers of GHG production.

Aquatic primary productivity produces oxygen (O₂) and consumes carbon dioxide (CO₂) in a ratio of ~1.2. However, in aquatic ecosystems, dissolved CO₂concentrations can be low, potentially limiting primary productivity. Here, results show that a large drainage basin maintains its highest levels of gross primary productivity (GPP) when dissolved CO₂is diminished or undetectable due to photosynthetic uptake. Data show that, after CO₂is depleted, bicarbonate, an ionized form of inorganic carbon, supports these high levels of productivity. In fact, outputs from a process‐based model suggest that bicarbonate can support up to ~58% of GPP under the most productive conditions. This is the first evidence that high levels of aquatic GPP are sustained in a riverine drainage network despite CO₂depletion, which has implications for freshwater ecology, biogeochemistry, and isotopic analysis.

Streams and rivers are significant sources of carbon dioxide (CO₂) and methane (CH₄) to the atmosphere. However, the magnitudes of these fluxes are uncertain, in part, because dissolved greenhouse gases (GHGs) can exhibit high spatiotemporal variability. Concentration‐discharge (C‐Q) relationships are commonly used to describe temporal variability stemming from hydrologic controls on solute production and transport. This study assesses how the partial pressures of two GHGs—pCO₂andpCH₄—vary across hydrologic conditions over 4 yr in eight nested streams and rivers, at both annual and seasonal timescales. Overall, the range ofpCO₂was constrained, ranging from undersaturated to nine times oversaturated, whilepCH₄was highly variable, ranging from 3 to 500 times oversaturated. We show thatpCO₂exhibited chemostatic behavior (i.e., no change withQ), in part, due to carbonate buffering and seasonally specific storm responses. In contrast, we show thatpCH₄generally exhibited source limitation (i.e., a negative relationship withQ), which we attribute to temperature‐mediated production. However,pCH₄exhibited chemostasis in a wetland‐draining stream, likely due to hydrologic connection to the CH₄‐rich wetland. These findings have implications for CO₂and CH₄fluxes, which are controlled by concentrations and gas transfer velocities. At highQ, enhanced gas transfer velocity acts on a relatively constant CO₂stock but on a diminishing CH₄stock. In other words, CO₂fluxes increase withQ, while CH₄fluxes are modulated by the divergentQdynamics of gas transfer velocity and concentration.