Significance Stream/river carbon dioxide (CO 2 ) emission has significant spatial and seasonal variations critical for understanding its macroecosystem controls and plumbing of the terrestrial carbon budget. We relied on direct fluvial CO 2 partial pressure measurements and seasonally varying gas transfer velocity and river network surface area estimates to resolve reach-level seasonal variations of the flux at the global scale. The percentage of terrestrial primary production (GPP) shunted into rivers that ultimately contributes to CO 2 evasion increases with discharge across regions, due to a stronger response in fluvial CO 2 evasion to discharge than GPP. This highlights the importance of hydrology, in particular water throughput, in terrestrial–fluvial carbon transfers and the need to account for this effect in plumbing the terrestrial carbon budget.

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Lakes play an important role in the global carbon cycle, and littoral zones of lakes are potential hotspots of greenhouse gas production. In this study, we measured the partial pressures of carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) in the littoral zones of 17 lakes on the Tibetan Plateau. The littoral zones of lakes on the Tibetan Plateau were supersaturated and acted as sources of CO 2 , CH 4 , and N 2 O to the atmosphere. The average partial pressures of CO 2 , CH 4 , and N 2 O in the surface lake water were 664.8 ± 182.5, 139.8 ± 335.6, and 0.3 ± 0.1 μatm, respectively. The average diffusive fluxes (and uncentainty intervals) of these three gases were 73.7 (0.9-295.3) mmol · m À2 · day À1 , 5.2 (0.0008-45.9) mmol · m À2 · day À1 , and 6.5 (0.07-20.9) μmol · m À2 · day À1 , respectively. The diffusive fluxes of CO 2 in lakes were significantly correlated with dissolved organic carbon, dissolved organic nitrogen, salinity, and water temperature. The diffusive fluxes of N 2 O were significantly correlated with lake water depth. However, no relationships were found between environmental factors and the CH 4 diffusive flux at the scale of this study. CO 2 exchange with the atmosphere from saline lakes was found to be higher than from freshwater lakes with equivalent CO 2 concentrations by a factor of 2.5 due to chemical enhancement of the gas transfer velocity. Therefore, further study with enhanced spatiotemporal resolution and breadth is needed to better understand the important role played by lakes on the Tibetan Plateau in both regional and global carbon cycles.

Drought is common in rivers, yet how this disturbance regulates metabolic activity across network scales is largely unknown. Drought often lowers gross primary production (GPP) and ecosystem respiration (ER) in small headwaters but by contrast can enhance GPP and cause algal blooms in downstream estuaries. We estimated ecosystem metabolism across a nested network of 13 reaches from headwaters to the main stem of the Connecticut River from 2015 through 2017, which encompassed a pronounced drought. During drought, GPP and ER increased, but with greater enhancement in larger rivers. Responses of GPP and ER were partially due to warmer temperatures associated with drought, particularly in the larger rivers where temperatures during summer drought were > 10 C higher than typical summer baseflow. The larger rivers also had low canopy cover, which allowed primary producers to take advantage of lower turbidity and fewer cloudy days during drought. We conclude that GPP is enhanced by higher temperature, lower turbidity, and longer water residence times that are all a function of low discharge, but ecosystem response in temperate watersheds to these drivers depends on light availability regulated by riparian canopy cover. In larger rivers, GPP increased more than ER during drought, even leading to temporary autotrophy, an otherwise rare event in the typically light-limited heterotrophic Connecticut River main stem. With climate change, rivers and streams may become warmer and drought frequency and severity may increase. Such changes may increase autotrophy in rivers with broad implications for carbon cycling and water quality in aquatic ecosystems.Droughts are a disturbance to river ecosystems with farreaching effects on their structure and function. As freshwaters enter drought, the physical template (e.g., temperature, turbidity, conductivity, and pH) of stream and river ecosystems changes. These fundamental habitat alterations affect all components of the food web and the physicochemical environment (Stanley et al. 1997;Dahm et al. 2003). Many of the changes to river networks during drought are stressful to biota,

Greenhouse gas evasion from inland waters is a globally significant yet highly uncertain flux, especially in regard to effects of wetlands and hydrologic variability. We sampled five first‐order and two second‐order streams with variable wetland influence during storm events for dissolved CO2, CH4, and N2O. We also calculated gas evasion rates. In first‐order streams, pCO2 and pN2O were significantly higher in the stream with the most wetland influence (mean ± 1 std: 3,965 ± 1,504 and 1.18 ± 0.37 μatm, respectively) than the forested stream (2,927 ± 439 and 0.47 ± 0.08 μatm, respectively). In second‐order streams, pCO2, pCH4, and pN2O were higher in the 14% wetland stream (3,274 ± 825, 501 ± 207, and 1.37 ± 0.43 μatm, respectively) than in the 2% wetland stream (1,858 ± 423, 137 ± 53, and 0.37 ± 0.08 μatm, respectively). In first‐order streams, pCO2 in streams with wetland influence increased during rain events, while pCO2 in streams with little to no wetland influence decreased or remained constant. Generally, pCH4 and pN2O followed the same trend, except in one stream with intermediate wetland influence. Gas transfer velocity increased in all streams during storm events. However, the forested streams had higher gas transfer velocities than the wetland streams due to steeper topography. CO2, CH4, and N2O evasion peaked in one of the intermediate wetland streams at high flow (maximum: 66 g C·m−2·day−1, 177 mg C·m−2·day−1, and 9.7 mg N·m−2·day−1, respectively). These findings suggest that gases are shunted downstream in flatter, wetland streams, while gases are evaded closer to their source in steeper, forested streams.