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Rivers and streams are control points for CO₂emission to the air (fCO₂), with emission rates often exceeding internal metabolism (net ecosystem production, NEP). The difference is usually attributed to CO₂‐supersaturated groundwater inputs from upland soil respiration and rock weathering, but this implies a terrestrial‐to‐aquatic C transfer greater than estimated by terrestrial mass balance. One explanation is that riparian zones—rich in organic and inorganic C but mostly neglected in terrestrial mass balances—contribute disproportionately tofCO₂. To test this hypothesis, we measuredfCO₂, NEP, and the lateral CO₂contributions from both terrestrial uplands (TER) and riparian wetlands (RIP) for seven reaches in a lowland river network in Florida, USA. NEP contributed about half offCO₂, but the remaining CO₂emission was generally much larger than measured TER. The relative importance of RIP versus TER varied markedly between contrasting hydrogeologic settings: RIP contributed 49% offCO₂where geologic confinement forced lateral drainage through riparian soils, but only 12% where unconfined karst allowed deeper groundwater flowpaths that bypassed riparian zones. On a land area basis, the narrow riparian corridor yielded far more CO₂than the terrestrial uplands (33.1 vs. 1.4 g‐C m⁻² yr⁻¹), resulting in river corridors (i.e., stream channel plus adjacent wetlands, NEP + RIP) sourcing 87% offCO₂to streams. Our findings imply that true terrestrial CO₂subsidies to streams may be smaller than previously estimated by aquatic mass balance and highlight the importance of explicitly integrating riparian zones into the conceptual model for terrestrial‐to‐aquatic C transfer.

 

Mean annual temperature and mean annual precipitation drive much of the variation in productivity across Earth's terrestrial ecosystems but do not explain variation in gross primary productivity (GPP) or ecosystem respiration (ER) in flowing waters. We document substantial variation in the magnitude and seasonality of GPP and ER across 222 US rivers. In contrast to their terrestrial counterparts, most river ecosystems respire far more carbon than they fix and have less pronounced and consistent seasonality in their metabolic rates. We find that variation in annual solar energy inputs and stability of flows are the primary drivers of GPP and ER across rivers. A classification schema based on these drivers advances river science and informs management. 

Headwater streams are control points for carbon dioxide (CO₂) emissions to the atmosphere, with relative contributions to CO₂emission fluxes from lateral groundwater inputs widely assumed to overwhelm those from in‐stream metabolic processes. We analyzed continuous measurements of stream dissolved CO₂and oxygen (O₂) concentrations during spring and early summer in two Mediterranean headwater streams from which we evaluated the contribution of in‐stream net ecosystem production (NEP) to CO₂emission. The two streams exhibited contrasting hydrological regimes: one was non‐perennial with relatively small groundwater inflows, while the other was perennial and received significant lateral groundwater inputs. The non‐perennial stream exhibited strong inverse coupling between instantaneous and daily CO₂and O₂concentrations, and a strong correlation between aerobic ecosystem respiration (ER) and gross primary production (GPP) despite persistent negative NEP. At the perennial stream, the CO₂–O₂relationship varied largely over time, ER and GPP were uncorrelated, and NEP, which was consistently negative, increased with increasing temperature. Mean NEP contribution to CO₂emission was 51% and 57% at the non‐perennial and perennial stream, respectively. Although these proportions varied with assumptions about metabolic stoichiometry and groundwater CO₂concentration, in‐stream CO₂production consistently and substantially contributed to total atmospheric CO₂flux in both streams. We conclude that in‐stream metabolism can be more important for driving C cycling in some headwater streams than previously assumed.

 

Floods are dominant controls on export of solutes from catchments. In contrast, low‐flow periods such as droughts are potentially dominant control points for biogeochemical processing, enhancing spatiotemporal variation in solute concentrations, stream metabolism, and nutrient uptake. Using complementary time series (i.e., an Eulerian reference frame) and longitudinal profiling (i.e., a Lagrangian reference frame), we investigated hydrologic controls on temporal and spatial variation in solute flux and metabolism in the Lower Santa Fe River (FL, USA), where highly colored surface water mixes with exceptionally clear groundwater from springs. Gage measurements suggest groundwater inputs ranged from <1% (during extreme floods) to 86% (during extreme drought) of total discharge (Q). Mass transport of most solutes was dominated by high‐Qperiods. Most soluteC‐Qrelationships exhibited statistically significant slope breakpoints near the transition between surface and groundwater dominance. In particular, parameters controlling water column light attenuation were chemostatic above medianQbut markedly reduced at lowQ. As a result, river metabolism and assimilatory nitrate (NO₃⁻) uptake were consistently suppressed at highQand enhanced at lowQ, with greater variability in response to drivers other than water column light transmittance. Spatial variation in solute concentrations was also enhanced at lowQ, induced by discrete groundwater inflow and biogeochemical processing along the reach. Contrasting reference frames yielded corroborative evidence for transport dominance at highQ, which damps spatiotemporal heterogeneity. In contrast, low‐Qperiods enable localized mixing controls on solute concentrations and high rates of metabolism and nutrient processing that increase spatiotemporal variability.