Search for: All records

The magnitude of stream and river carbon dioxide (CO 2 ) emission is affected by seasonal changes in watershed biogeochemistry and hydrology. Global estimates of this flux are, however, uncertain, relying on calculated values for CO 2 and lacking spatial accuracy or seasonal variations critical for understanding macroecosystem controls of the flux. Here, we compiled 5,910 direct measurements of fluvial CO 2 partial pressure and modeled them against watershed properties to resolve reach-scale monthly variations of the flux. The direct measurements were then combined with seasonally resolved gas transfer velocity and river surface area estimates from a recent global hydrography dataset to constrain the flux at the monthly scale. Globally, fluvial CO 2 emission varies between 112 and 209 Tg of carbon per month. The monthly flux varies much more in Arctic and northern temperate rivers than in tropical and southern temperate rivers (coefficient of variation: 46 to 95 vs. 6 to 12%). Annual fluvial CO 2 emission to terrestrial gross primary production (GPP) ratio is highly variable across regions, ranging from negligible (<0.2%) to 18%. Nonlinear regressions suggest a saturating increase in GPP and a nonsaturating, steeper increase in fluvial CO 2 emission with discharge across regions, which leads to higher percentages of GPP being shunted into rivers for evasion in wetter regions. This highlights the importance of hydrology, in particular water throughput, in routing terrestrial carbon to the atmosphere via the global drainage networks. Our results suggest the need to account for the differential hydrological responses of terrestrial–atmospheric vs. fluvial–atmospheric carbon exchanges in plumbing the terrestrial carbon budget. 

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.

 

River networks transport dissolved organic carbon (DOC) from terrestrial uplands to the coastal ocean. The extent to which a reach or lake within a river network uptakes DOC depends on the stream order, the seasonal conditions, and the flow. At the watershed scale, it remains unclear whether DOC uptake is dominated by biological processes such as respiration, or abiotic processes like photomineralization. The partitioning of DOC uptake in lakes vs. rivers is also unclear. In this study, we present a new model that unifies year‐round controls on DOC cycling for an entire river network, including river–lake connectivity, to elucidate the importance of biotic vs. abiotic controls on DOC uptake. We present the Catchment UPtake and Sinks by Season, Order, and Flow for DOC (CUPS‐OF‐DOC) model, which quantifies terrestrial DOC loading, gross primary productivity, and uptake via microbes and photomineralization. The model is applied to the Connecticut River Watershed, and accounts for cascading reach‐ and lake‐scale DOC cycling across 98 scenarios spanning combinations of flows, seasons, and stream orders. We show that riverine DOC uptake is nearly constant with stream order, but the proportion of DOC uptake from photomineralization varies. Photomineralization dominates in rivers in most flow conditions and stream orders, especially in winter, accounting for at least half of whole‐watershed DOC uptake in February across all flows. Whole‐watershed summer DOC uptake occurs mostly via biomineralization in lakes, accounting for 80% of DOC uptake during the growing season, despite accounting for less than 6% of watershed open water surface area.

 

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.

 

Sunlight can oxidize dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) in freshwaters. The importance of complete photooxidation, or photomineralization, as a sink for DOC remains unclear in temperate rivers, as most estimates are restricted to lakes, high latitude rivers, and coastal river plumes. In this study, we construct a model representing over 75,000 river reaches in the Connecticut River Watershed (CRW), USA, to calculate spectrally resolved photomineralization. We test the hypothesis that photomineralization is a negligible DOC sink across all reaches and flow conditions relative to DOC fluxes. Our model quantifies reaction rates and transport drivers within the river reaches for the ranges of flow conditions, incoming solar irradiance, and canopy cover shading observed throughout the year. Our model predicts average daily areal photomineralization rates ranging from 1.16 mg‐C m⁻² day⁻¹in low flow river reaches in the winter, to 18.33 mg‐C m⁻² day⁻¹in high flow river reaches during the summer. Even for high photomineralization fluxes, corresponding photomineralization uptake velocities are typically at least an order of magnitude smaller than those reported for other instream processes. We calculate DOC elimination by photomineralization relative to DOC fluxes through individual stream reaches as well as the entire riverine portion of the CRW. We find that relative photomineralization fluxes are highest in summer drought conditions in low order streams. In median flows and mean light intensities, for an average watershed travel distance, 3%–5% of the DOC fluxes are eliminated, indicating that photomineralization is a minor DOC sink in temperate rivers.

 

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.