A large fraction of the organic carbon derived from land that is transported through inland waters is decomposed along river systems and emitted to the atmosphere as carbon dioxide (CO 2 ). The Amazon River outgasses nearly as much CO 2 as the rainforest sequesters on an annual basis, representing ∼25% of global CO 2 emissions from inland waters. However, current estimates of CO 2 outgassing from the Amazon basin are based on a conservative upscaling of measurements made in the central Amazon, meaning both basin and global scale budgets are likely underestimated. The lower Amazon River, from Óbidos to the river mouth, represents ∼13% of the total drainage basin area, and is not included in current basin-scale estimates. Here, we assessed the concentration and evasion rate of CO 2 along the lower Amazon River corridor and its major tributaries, the Tapajós and Xingu Rivers. Evasive CO 2 fluxes were directly measured using floating chambers and gas transfer coefficients (k 600 ) were calculated for different hydrological seasons. Temporal variations in pCO 2 and CO 2 emissions were similar to previous observations throughout the Amazon (e.g., peak concentrations at high water) and CO 2 outgassing was lower in the clearwater tributaries compared to the mainstem. However, k 600 -values were higher than previously reported upstream likely due to the generally windier conditions, turbulence caused by tidal forces, and an amplification of these factors in the wider channels with a longer fetch. We estimate that the lower Amazon River mainstem emits 0.2 Pg C year −1 within our study boundaries, or as much as 0.48 Pg C year −1 if the entire spatial extent to the geographical mouth is considered. Including these values with updated basin scale estimates and estimates of CO 2 outgassing from small streams we estimate that the Amazon running waters outgasses as much as 1.39 Pg C year −1 , increasing the global emissions from inland waters by 43% for a total of 2.9 Pg C year −1 . These results highlight a large missing gap in basin-scale carbon budgets along the complete continuum of the Amazon River, and likely most other large river systems, that could drastically alter global scale carbon budgets.

Here we present direct measurements of the biological breakdown of 13C‐labeled substrates to CO2 at seven locations along the lower Amazon River, from Óbidos to the mouth. Dark incubation experiments were performed at high and low water periods using vanillin, a lignin phenol derived from vascular plants, and at the high water period using four different 13C‐labeled plant litter leachates. Leachates derived from oak wood were degraded most slowly with vanillin monomers, macrophyte leaves, macrophyte stems, and whole grass leachates being converted to CO2 1.2, 1.3, 1.7, and 2.3 times faster, respectively, at the upstream boundary, Óbidos. Relative to Óbidos, the sum degradation rate of all four leachates was 3.3 and 2.6 times faster in the algae‐rich Tapajós and Xingu Rivers, respectively. Likewise, the leachates were broken down 3.2 times more quickly at Óbidos when algal biomass from the Tapajós River was simultaneously added. Leachate reactivity similarly increased from Óbidos to the mouth with leachates breaking down 1.7 times more quickly at Almeirim (midway to the mouth) and 2.8 times more quickly across the river mouth. There was no discernible correlation between in situ nutrient levels and remineralization rates, suggesting that priming effects were an important factor controlling reactivity along the continuum. Further, continuous measurements of CO2, O2, and conductivity along the confluence of the Tapajós and Amazon Rivers and the Xingu and Jarauçu Rivers revealed in situ evidence for enhanced O2 drawdown and CO2 production along the mixing zone of these confluences.

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO 2 supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH 4 ) flux, direct CO 2 and CH 4 fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO 2 concentrations ranged from 6,491 to 14,976 latm and directly-measured stream CO 2 outgassing flux was 5,994 ± 677 g C m -2 y -1 of stream surface. Stream pCH 4 concentrations ranged from 291 to 438 latm and measured stream CH 4 outgassing flux was 987 ± 221 g C m -2 y -1 . Despite high flux rates from the stream surface, the small area of stream itself (970 m 2 , or 0.007% of watershed area) led to small directly-measured annual fluxes of CO 2 (0.44 ± 0.05 g C m 2 y -1 ) and CH 4 (0.07 ± 0.02 g C m 2 y -1 ) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m -2 y -1 ), DOC (0.16 ± 0.03 g C m -2 y -1 ) and coarse plus fine particulate C (0.001 ± 0.001 g C m -2 y -1 ) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m -2 y -1 as CO 2 outgassing, 11.29 g C m -2 y -1 as fluvial DIC and 0.64 g C m -2 y -1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40-50 g C m -2 y -1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m -2 y -1 ), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m -2 (±1.5 Mg C ha -1 ) that is exchanged annually across Amazon tropical forest canopies.

Most measurements of respiration rates in large tropical rivers do not account for the influence of river flow conditions on microbial activity. We developed a ship-board spinning incubation system for measuring O 2 drawdown under different rotation velocities and deployed the system along the lower Amazon River during four hydrologic periods. Average respiration rates in incubation chambers rotated at 0.22 m s 21 and 0.66 m s 21 were 1.4 and 2.4 times higher than stationary chambers, respectively. On average, depth-integrated Scientific Significance StatementMost measurements of aquatic respiration in large rivers ignore the influence of river flow on reaction rates, which we hypothesize has resulted in an underestimation of the contribution of microbial respiration to CO 2 outgassing in large tropical rivers. Here, we evaluate microbial respiration rates along the lower Amazon River under different simulated flow conditions using rotating incubation chambers. We demonstrated that river velocity and hydrodynamic conditions are key factors controlling microbial metabolism of river-borne organic matter, and that microbial respiration can potentially exceed CO 2 outgassing rates in the Amazon River mainstem and be balanced by primary production, outgassing, and other inputs. 265Limnology and Oceanography Letters 3, 2018, 265-274

The Amazon River outgasses nearly an equivalent amount of CO 2 as the rainforest sequesters on an annual basis due to microbial decomposition of terrigenous and aquatic organic matter. Most research performed in the Amazon has been focused on unraveling the mechanisms driving CO 2 production since the recognition of a persistent state of CO 2 supersaturation. However, although the river system is clearly net heterotrophic, the interplay between primary production and respiration is an essential aspect to understanding the overall metabolism of the ecosystem and potential transfer of energy up trophic levels. For example, an efficient ecosystem is capable of both decomposing high amounts of organic matter at lower trophic levels, driving CO 2 emissions, and accumulating energy/biomass in higher trophic levels, stimulating fisheries production. Early studies found minimal evidence for primary production in the Amazon River mainstem and it has since been assumed that photosynthesis is strongly limited by low light penetration attributed to the high sediment load. Here, we test this assumption by measuring the stable isotopic composition of O 2 (δ 18 O-O 2 ) and O 2 saturation levels in the lower Amazon River from Óbidos to the river mouth and its major tributaries, the Xingu and Tapajós rivers, during high and low water periods. An oxygen mass balance model was developed to estimate the input of photosynthetic oxygen in the discrete reach from Óbidos to Almeirim, midway to the river mouth. Based on the oxygen mass balance we estimate that primary production occurred at a rate of 0.39 ± 0.24 g O m 3 d −1 at high water and 1.02 ± 0.55 g O m 3 d −1 at low water. This translates to 41 ± 24% of the rate of O 2 drawdown via respiration during high water and 67 ± 33% during low water. These primary production rates are 2-7 times higher than past estimates for the Amazon Gagne-Maynard et al.Amazon River Oxygen Balance River mainstem. It is possible that at high water much of this productivity signal is the result of legacy advection from floodplains, whereas limited floodplain connectivity during low water implies that most of this signal is the result of in situ primary production in the Amazon River mainstem.

As a raindrop falls from the atmosphere, over vegetation and forest canopies, and enters soils and streams it experiences a dynamic exchange of carbon constituents with the surrounding environment. Understanding the magnitude and dynamics of carbon export in above and below ground flow paths is critical for constraining the influence of terrestrial and aquatic ecosystems on global carbon cycling. Here we examine the concentration and flux of dissolved organic and inorganic carbon (DOC and DIC) in rainfall, throughfall, stemflow, overland flow, soil solution, groundwater, and stream water in an Amazonian transitional forest located near the arc of deforestation. Rainfall was enriched in DOC by interactions with atmospheric particles derived from both biogenic and anthropogenic emissions, resulting in a flux of 82.3 kg C ha −1 yr −1 , which was the largest flux from each respective flow path. Forest throughfall, stemflow, and direct overland flow mobilized DOC from products of terrestrial primary production and decomposition. Net throughfall represented the second largest DOC flux (68.4 kg C ha −1 yr −1 ), whereas stemflow and overland flow only had a flux of 1.5 and 3.9 kg C ha −1 yr −1 , respectively. Much of the DOC in above ground flow paths was removed from solution as the rain percolated through soil layers due to both biological decomposition and sorption/desorption to mineral surfaces, resulting in low concentrations in stream and groundwater (2.6 ± 2.4 mg L −1 and 1.45 ± 0.3 mg L −1 , respectively) relative to throughfall (43.9 ± 5.2 mg L −1 ) and stemflow (30.6 ± 2.7 mg L −1 ). The flux of DIC in each respective flow path was generally lower than for DOC, and likely driven by atmospheric gas exchange and inputs from respiration and decomposition. DOC concentrations in above ground flow paths were highest during the first rainfall after a dry period, whereas DIC concentrations generally increased throughout the rainy season as soil moisture increased.

Coloured dissolved organic matter (CDOM) is one of the major contributors to the absorption budget of most freshwaters and can be used as a proxy to assess non-optical carbon fractions such as dissolved organic carbon (DOC) and the partial pressure of carbon dioxide (pCO). Nevertheless, riverine studies that explore the former relationships are still relatively scarce, especially within tropical regions. Here we document the spatial-seasonal variability of CDOM, DOC and pCO, and assess the potential of CDOM absorption coefficient (a(412)) for estimating DOC concentration and pCO along the Lower Amazon River. Our results revealed differences in the dissolved organic matter (DOM) quality between clearwater (CW) tributaries and the Amazon River mainstream. A linear relationship between DOC and CDOM was observed when tributaries and mainstream are evaluated separately (Amazon River: N = 42, R = 0.74, p<0.05; CW: N = 13, R = 0.57, p<0.05). However, this linear relationship was not observed during periods of higher rainfall and river discharge, requiring a specific model for these time periods to be developed (N = 25, R = 0.58, p<0.05). A strong linear positive relation was found between a(412) and pCO(N = 69, R = 0.65, p<0.05) along the lower river. pCO was less affected by the optical difference between tributaries and mainstream waters or by the discharge conditions when compared to CDOM to DOC relationships. Including the river water temperature in the model improves our ability to estimate pCO (N = 69; R = 0.80, p<0.05). The ability to assess both DOC and pCO from CDOM optical properties opens further perspectives on the use of ocean colour remote sensing data for monitoring carbon dynamics in large running water systems worldwide.

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