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.

Carbon dioxide (CO2) emissions to the atmosphere from running waters are estimated to be four times larger than the total carbon (C) flux to the oceans. However, these fluxes remain poorly constrained because of substantial temporal variability in dissolved CO2 concentrations. Using a global compilation of high frequency CO2 measurements, we demonstrate that nocturnal CO2 emissions are consistently larger, by an average of 27% (0.9 g C m -2 d -1 ), than those estimated from diurnal concentrations alone. Canopy shading is the principal control on observed diel (24 hr) variation, suggesting this nocturnal increase arises from daytime fixation of dissolved inorganic C by photosynthesis. Because contemporary global estimates of CO2 emissions to the atmosphere from running waters (0.65 -1.8 Pg C yr -1 ) rely primarily on discrete measurements of dissolved CO2 obtained during the day, they substantially underpredict the magnitude of this important flux. Accounting for night-time CO2 elevates global estimates of emissions from running waters to the atmosphere by 0.20-0.55 Pg C yr -1 .Carbon dioxide (CO2) emission from inland waters to the atmosphere is a major flux in the global carbon (C) cycle, and four-fold larger than the lateral C export to oceans 1 . Streams and rivers are hotspots for this flux, accounting for ~85% of inland water CO2 emissions despite covering <20% of the freshwater surface area 2 . Despite this importance, the magnitude of global CO2 emissions from streams and rivers remains highly uncertain with estimates revised upwards over the past decade from 0.6 to 3.48 Pg C yr -1 (3,4) . Changes to this estimate follow improvements in the spatial resolution for upscaling emissions 2,5 , as well as new studies from previously underrepresented areas such as the Congo 6 , Amazon 7 , and global mountains 8 . Further refinements have emerged from considering temporal variability in CO2 emission rates 9 . However, despite recent studies showing dramatic day-night changes in stream and river water CO2 concentrations 10-14 the significance of systematic sub-daily variation on overall CO2 emissions remains unexplored.Diurnal cycles in solar radiation impose a well-known periodicity on stream biogeochemical processes, creating diel (i.e., 24-hr period lengths) patterns for many solutes and gases, including nutrients, dissolved organic matter, and dissolved oxygen (O2) 15 . Indeed, diel variation in O2 arising from photosynthetic activity is the signal from which whole-system metabolic fluxes are estimated 16 . Photosynthetic production of O2 is stoichiometrically linked to the day-time assimilation of dissolved inorganic carbon (principally bicarbonate and dissolved CO2), lowering CO2 concentrations during the day. The resulting diel variation, with higher night-time CO2 concentrations when respiration reactions dominate, implies increased emissions at night. Despite the obvious connection between photosynthesis and CO2 consumption, the implications for total aquatic CO2 emissions has been neglected, most likely ...

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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‐Q periods. Most solute C‐Q relationships exhibited statistically significant slope breakpoints near the transition between surface and groundwater dominance. In particular, parameters controlling water column light attenuation were chemostatic above median Q but markedly reduced at low Q. As a result, river metabolism and assimilatory nitrate (NO3−) uptake were consistently suppressed at high Q and enhanced at low Q, with greater variability in response to drivers other than water column light transmittance. Spatial variation in solute concentrations was also enhanced at low Q, induced by discrete groundwater inflow and biogeochemical processing along the reach. Contrasting reference frames yielded corroborative evidence for transport dominance at high Q, which damps spatiotemporal heterogeneity. In contrast, low‐Q periods enable localized mixing controls on solute concentrations and high rates of metabolism and nutrient processing that increase spatiotemporal variability.

Headwater streams are control points for carbon dioxide (CO 2 ) emissions to the atmosphere, with relative contributions to CO 2 emission fluxes from lateral groundwater inputs widely assumed to overwhelm those from in-stream metabolic processes. We analyzed continuous measurements of stream dissolved CO 2 and oxygen (O 2 ) 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 CO 2 emission. 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 CO 2 and O 2 concentrations, and a strong correlation between aerobic ecosystem respiration (ER) and gross primary production (GPP) despite persistent negative NEP. At the perennial stream, the CO 2 -O 2 relationship varied largely over time, ER and GPP were uncorrelated, and NEP, which was consistently negative, increased with increasing temperature. Mean NEP contribution to CO 2 emission was 51% and 57% at the non-perennial and perennial stream, respectively. Although these proportions varied with assumptions about metabolic stoichiometry and groundwater CO 2 concentration, in-stream CO 2 production consistently and substantially contributed to total atmospheric CO 2 flux in both streams. We conclude that in-stream metabolism can be more important for driving C cycling in some headwater streams than previously assumed.Streams and rivers are control points in global carbon (C) cycling because of high carbon dioxide (CO 2 ) emission rates (Caraco and Cole 2003;Cole et al. 2007;Drake et al. 2018). High emissions from small streams are attributed principally to groundwater inputs of dissolved CO 2 from soil organic matter decomposition (