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Sampling site location in the Amazon basin. Each site was sampled at different times of the year from 1994 to 1996
Source publication
The chemical composition of the Amazon River results from the mixing of two water types: black water and white water. On-site fractionation by sequential tangential ultrafiltration (STUF) was used to differentiate transported organic carbon and to determine the distribution and association of major and trace elements with different size fraction of...
Contexts in source publication
Context 1
... were taken from 1994 to 1997. Sampling sites are the Manacapurù station for the Rio Solimões, the Paricatuba station for the Rio Negro, 20 km upstream of the confluence with the Amazon for the Rio Madeira, and 496 km downstream of this last station, at thè Obidos station, for the mixed sample on the Amazon (Figure 1). Surface water samples (0-0Ð5 m) were taken and treated on board with sequential tangential flow ultrafiltration (ultrasart from Sartorius). ...
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... the Rio Solimões and the Amazon downstream of Manaus more than 75% of the bulk OC is OC D , except in March 1995 when it accounts for only 50%. Thus the carbon flux at`Obidosat` at`Obidos (station 4 in Figure 1), once the mixing between major tributaries is completed, is essentially due to organic carbon smaller than 5000 Da. The larger contribution of colloidal carbon in March 1995 for all rivers could be due to the fact that it was an exceptionally dry year. ...
Context 3
... annual carbon fluxes and size distribution were evaluated assuming that the relations were linear between sample collection periods. The hydrological budget at thè Obidos station (station 4 in Figure 1) was evaluated; the discharges for the Rio Negro, Rio Madeira and Rio Solimões were summed and compared to the Amazon at`Obidosat` at`Obidos ( Figure 6A). The hydrological balance varied from 5% to 20% assuming a maximum error of 10% in the discharge as measured by the ADCP (Guyot et al., 1998). ...
Citations
... These DOC-rich waters harbor relatively high concentrations of metals (Fe, Al, Zn) reflecting the preferential binding of metals to humic substances 7 and the changes to metal speciation associated with low pH. However, the binding properties of the silicate sand found at the bottom of these habitats strips the water from its major cations (Na + , Cl − , Mg 2+ , and Ca 2+ ), leaving it extremely ion-poor 7 . Overall, the chemistry of these blackwaters appears so extreme that they would most likely be toxic to most freshwater fish, yet the ichthyofauna inhabiting these habitats is one of the most biodiverse on Earth, with an estimated richness of 1700 species 8 , surpassing the estimated 1620 fish species on the Great Barrier Reef 9 . ...
Amazonian blackwaters are extremely biodiverse systems containing some of Earth’s most naturally acidic, dissolved organic carbon -rich and ion‐poor waters. Physiological adaptations of fish facing these ionoregulatory challenges are unresolved but could involve microbially-mediated processes. Here, we characterize the physiological response of 964 fish-microbe systems from four blackwater Teleost species along a natural hydrochemical gradient, using dual RNA-Seq and 16 S rRNA of gill samples. We find that host transcriptional responses to blackwaters are species-specific, but occasionally include the overexpression of Toll-receptors and integrins associated to interkingdom communication. Blackwater gill microbiomes are characterized by a transcriptionally-active betaproteobacterial cluster potentially interfering with epithelial permeability. We explore further blackwater fish-microbe interactions by analyzing transcriptomes of axenic zebrafish larvae exposed to sterile, non-sterile and inverted (non-native bacterioplankton) blackwater. We find that axenic zebrafish survive poorly when exposed to sterile/inverted blackwater. Overall, our results suggest a critical role for endogenous symbionts in blackwater fish physiology.
... Different size fractions of the bulk DOM have been shown to have different spatial and temporal distribution and variation patterns and different biogeochemical properties and reactivities (Engbrodt & Kattner, 2005;Zhou, Stolpe, et al., 2016). In addition, sizes of DOM may play a critical role in regulating the concentration and speciation, and hence the fate, transport, and bioavailability of trace metals and pollutants in aquatic systems (Aiken et al., 2011;Benedetti et al., 2003;Lead & Wilkinson, 2006;Stolpe et al., 2010;Wang et al., 2017;Zhou, Stolpe, et al., 2016). The size of DOM itself may also change greatly with time, during the transformation under the influence of various chemical and biological processes, such as microbial activities and photobleaching (Benner & Amon, 2015;Zhu, Zhang, & Yang, 2018). ...
Large world rivers and their dissolved organic matter (DOM) fluxes could regulate ecosystem function and biogeochemical processes in coastal marine environments. Knowledge about the seasonal variations in composition and molecular size of DOM remains scarce but is important to a better understanding of the role of river fluxes. Monthly time‐series (July 2018 to June 2019) surface water samples were collected from the lower Changjiang (Yangtze) River, one of the largest world rivers, for the measurements of both dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) to elucidate seasonal changes in their abundance, optical properties, and molecular size distributions among the <1 kDa, 1–3 kDa, 3–10 kDa, and 10 kDa–0.7 μm size fractions. On average, the <1 kDa low‐molecular weight DOM made up about two thirds of the bulk CDOM, leaving the other one third in the 1 kDa–0.7 μm size fraction. Optical properties and molecular size of DOM transported by the Changjiang River had an evident seasonality. In general, DOM exported from the Changjiang River can be characterized as lower DOC, lower aromaticity, and lower colloidal abundance compared to other world rivers. Together with available literature data, our results show that world rivers with higher/lower DOC concentrations and higher/lower aromaticity also contain more/less colloidal (>1 kDa) DOM. Our year‐long time‐series study provides additional valuable baseline data for a better understanding of seasonal changes in the abundance, composition, and molecular weight distribution of DOM from the lower Changjiang River and the potential impacts on the biogeochemical cycling of DOM in the adjacent estuarine, coastal, and shelf areas.
... The Usumacinta River discharge to the Gulf of Mexico increased by five times in the RS compared to the DS. This increase is noteworthy compared to that of other large rivers, such as the Amazon, Congo, Tana, Yellow, and Mississippi, which exhibited lower seasonal differences from 1.2 to 3.2 times [44,[51][52][53][54]. ...
Particulate organic carbon (POC) derived from inland water plays an important role in the global carbon (C) cycle; however, the POC dynamic in tropical rivers is poorly known. We assessed the POC concentration, flux, and sources in the Usumacinta, the largest tropical river in North America, to determine the controls on POC export to the Gulf of Mexico. We examined the Mexican middle and lower Usumacinta Basin during the 2017 dry (DS) and rainy (RS) seasons. The POC concentration ranged from 0.48 to 4.7 mg L−1 and was higher in the RS, though only in the middle basin, while remaining similar in both seasons in the lower basin. The POC was predominantly allochthonous (54.7 to 99.6%). However, autochthonous POC (phytoplankton) increased in the DS (from 5.1 to 17.7%) in both basins. The POC mass inflow–outflow balance suggested that floodplains supply (C source) autochthonous POC during the DS while retaining (C sink) allochthonous POC in the RS. Ranging between 109.1 (DS) and 926.1 t POC d−1 (RS), the Usumacinta River POC export to the Gulf of Mexico was similar to that of other tropical rivers with a comparable water discharge. The extensive floodplains and the “Pantanos de Centla” wetlands in the lowlands largely influenced the POC dynamics and export to the southern Gulf of Mexico.
... The blackwaters of the Rio Negro, a major tributary of the Amazon River, are characteristically dilute yet deeply colored (Val & Almeida-Val, 1995). The soil surrounding the river is largely composed of silicate sand and due to its binding properties, has been long since been stripped of its major cations, leaving the water in this region extremely ion-poor (Benedetti et al., 2003;Seyler & Boaventura, 2003). The unique geochemistry of the headwaters and breakdown of jungle vegetation are reflected in the water composition, making Rio Negro blackwaters some of the most naturally acidic, DOC-rich, and ion-poor waters anywhere in the world (Furch, 1984;Val & Almeida-Val, 1995;Walker & Henderson, 1996). ...
... DOC-rich waters that drain from the forest soils, rocks, and the plants growing there may have high concentrations of metals, reflecting the preferential binding of metals to humic substances (Benedetti et al., 2003;Seyler & Boaventura, 2003). Rainfall is an additional source of metals to Amazonian river systems which may partially explain the relatively high concentrations of Al, Sn, Se, Th, Rb, and Cd in the Rio Negro (Konhauser et al., 1994). ...
Although blackwaters, named for their rich content of dissolved organic carbon (DOC), are often very poor in ions and very acidic, they support great fish biodiversity. Indeed, about 8% of all freshwater fish species live in the blackwaters of the Rio Negro watershed in the Amazon basin. We review how native fish survive these harsh conditions that would kill most freshwater fish, with a particular focus on the role of DOC, a water quality parameter that has been relatively understudied. DOC, which is functionally defined by its ability to pass through a 0.45‐µm filter, comprises a diverse range of compounds formed by the breakdown of organic matter and is quantified by its carbon component that is approximately 50% by mass. Adaptations of fish to acidic blackwaters include minimal acid–base disturbances associated with a unique, largely unknown, high‐affinity Na⁺ uptake system that is resistant to inhibition by low pH in members of the Characiformes, and very tight regulation of Na⁺ efflux at low pH in the Cichliformes. Allochthonous (terrigenous) DOC, which predominates in blackwaters, consists of larger, more highly colored, reactive molecules than autochthonous DOC. The dissociation of protons from allochthonous components such as humic and fulvic acids is largely responsible for the acidity of these blackwaters, yet at the same time, these components may help protect organisms against the damaging effects of low water pH. DOC lowers the transepithelial potential (TEP), mitigates the inhibition of Na⁺ uptake and ammonia excretion, and protects against the elevation of diffusive Na⁺ loss in fish exposed to acidic waters. It also reduces the gill binding and toxicity of metals. At least in part, these actions reflect direct biological effects of DOC on the gills that are beneficial to ionoregulation. After chronic exposure to DOC, some of these protective effects persist even in the absence of DOC. Two characteristics of allochthonous DOC, the specific absorbance coefficient at 340 nm (determined optically) and the PBI (determined by titration), are indicative of both the biological effectiveness of DOC and the ability to protect against metal toxicity. Future research needs are highlighted, including a greater mechanistic understanding of the actions of DOCs on gill ionoregulatory function, morphology, TEP, and metal toxicity. These should be investigated in a wider range of native fish Orders that inhabit one of the world's greatest biodiversity hotspots for freshwater fishes.
Research Highlights
• Dissolved organic carbon (DOC) mitigates metal toxicity and provides ionoregulatory protection to freshwater fish, particularly in ion‐poor, acidic waters.
• The role of DOC in this protection is reviewed and future research needs are highlighted.
... La présence des ETMM transforment les sédiments en ultime puits mais aussi en une source secondaire de contaminants pour l'écosystème, au travers de la diffusion et de perturbations physiques naturelles ou anthropiques (Pan and Wang, 2012;Rigaud et al., 2013;Shaw et al., 1990;Singh et al., 2005). La répartition des ETMM, leur mobilité ainsi que leurs formes chimiques sont contrôlées par des processus physiques (diffusion, remise en suspension, advection…), chimiques (dissolution/précipitation, complexation, sorption…) et biologiques (bioirrigation, bioturbation, respiration…) (Elbaz-Poulichet, 2005;Rigaud et al., 2013;Scholz and Neumann, 2007;Tankere-Muller et al., 2007 (Benedetti et al., 2003;Dupré et al., 1999;Elbaz-Poulichet et al., 1999). Ces éléments peuvent passer de la phase liquide à la phase solide ou à la matrice biologique par des processus de sorption, de complexation ou d'assimilation biologique, tandis qu'inversement, ils passent en phase aqueuse suite aux processus de désorption et de dissolution. ...
La mer Méditerranée (MS) est une mer semi-fermée divisée en deux bassins: le bassin occidental et le bassin oriental. Les deux sites méditerranéens étudiés dans cette thèse sont au nord-ouest de MS et sud-est de France pour la rade de Toulon et à l'est de MS, Liban pour la baie de St-Georges. Les deux sites sont soumis à une forte densité de population le long de la côte et, sont exposés à de nombreuses activités anthropiques (telles que le tourisme, les transports maritimes, l’aquaculture, les rejets d’eaux usées et des activités industrielles) contaminant l’environnement (colonne d’eau, sédiments, biote, etc.) avec différents contaminants tels que les éléments traces métalliques et métalloïdes (ETMM). Chaque site est caractérisé par la présence d'un fleuve urbanisé: le Las (France) et le fleuve Beyrouth (Liban). Les sédiments sont considérés comme une source secondaire de contamination en raison des processus biogéochimiques influençant la mobilité des ETMM dans les sédiments. Dans ce contexte, les objectifs de cette thèse sont (1) d’étudier la contamination des ETMM le long des rivières et dans les deux baies; (2) d'élucider l'influence de la diagenèse précoce sur la mobilité des ETMM; et (3) de caractériser la matière organique sédimentaire dans les deux sites. Pour ces raisons, des sédiments superficiels, des carottes sédimentaires, des eaux superficielles et interstitielles ont été collectés le long des deux rivières et dans les deux sites durant la période 2016-2018. Les échantillons de sédiments et d'eaux ont été analysés pour déterminer leurs principaux paramètres physiques, nutriments, carbone organique (dissous et particulaire) et les éléments majeurs/traces. Les résultats ont montré que les processus de la diagenèse précoce contrôlent fortement la mobilité de la ETMM dans les sédiments. De plus, les résultats ont confirmé que la rade de Toulon est fortement contaminée suite à la deuxième guerre mondiale mais aussi aux activités contemporaines, et que les apports du Las ne contribuent et n’affectent pas de manière significative les sédiments de la baie de Toulon. Quant à la baie de St-Georges, il a été constaté qu’elle est impactée par plusieurs activités (effluents industriels rejetés sans traitement, ruissellement de la zone agricole) transportées par le fleuve Beyrouth et/ou par des apports directs (décharge côtière).
... Moreover, those studies do not try to relate changes in concentration or size of Ag-NPs to bulk geochemical parameters (i.e., pH, dissolved organic carbon concentration, ionic strength, redox …) of the aquatic ecosystems. As for any other type of natural colloids (Benedetti et al., 2003b;Ilina et al., 2016), it is also expected that the Ag-NPs flux will vary along the river and with the hydrological and climatic variations during the year. This knowledge is critical to build better material flow analysis models that can take into account such variations if they exist, and give outputs that can be compared to real field data like the one collected in this work. ...
This study focused on surface waters from three small creeks, within the Seine River watershed, which are characterized by different land-uses, namely forested, agricultural and urban. Silver nanoparticles (Ag-NPs) in these waters were detected and quantified by single-particle ICPMS during one-year of monthly sampling. Their temporal and spatial variations were investigated. Ag-NPs, in the three types of surface water, were found to range from 1.5 × 10⁷ to 2.3 × 10⁹ particles L⁻¹ and from 0.4 to 28.3 ng L⁻¹ at number and mass concentrations, respectively. These values are in consistent with the very few previous studies.
In addition, the role of factors driving process and potential sources are discussed with correlations between Ag-NPs concentrations and biogeochemical parameters, like dissolved organic carbon concentration and divalent cations concentrations. For the forested watershed NOM controls the stability (number and mass) of the Ag-NPs as recently observed in the field in lake water in Germany. In the case of the agricultural and urban watersheds major cations such as Ca would control the number and mass of Ag-NPs. Dilution processes are rejected as conductivity and Cl⁻ ions do not show significant correlations with Ag-NPs or other major geochemical parameters. The specific exportation rates of Ag-NPs for artificial, agricultural and forested areas were calculated based on the monthly data for the full year and are equal to 5.5 ± 3.0, 0.5 ± 0.3 and 0.2 ± 0.2 gy⁻¹km⁻², respectively. These data suggest a constant release of Ag-NPs from consumer products into freshwaters in artificial areas, for instance, from textiles, washing machines, domestic tap-water filters, outdoor paints.
These first data of Ag-NPs fluxes in surface waters of France enlarge the very limited database of field measurements. Moreover, for the first time, the influence of time, land-use and aquatic geochemistry parameters on Ag-NPs in real natural water samples is reported. It is also helpful to further understand the fate and the process of Ag-NPs in natural waters, as well as to the ecotoxicity studies in real-world environment.
... The different behaviour of these trace elements suggests processes other than simple adsorption/complexation by organic and iron colloids, such as precipitation as hydroxides and/or adsorption onto larger DOM aggregates or phyllosilicate clays. The precipitation of iron oxyhydroxide particles during mixing has been previously hypothesized as a mechanism for the removal of trace elements at the confluence of rivers (Benedetti et al., 2003;Steinmann and Stille, 2008). However, limited mixing up to 100 km downstream from confluences has also been observed (Bouchez et al., 2010). ...
Organic and inorganic colloids play important roles governing the speciation, transport, and bioaccessibility of trace elements in aquatic systems. These carriers are especially important in the boreal zone, where rivers that contain high concentrations of iron and organic matter are prevalent. The distribution of trace elements amongst different colloidal species (or “speciation profile”) can therefore be useful as a fingerprint to detect
different trace element sources and for tracking colloid transformations, with implications for bioaccessibility. Asymmetrical flow field-flow fractionation coupled to an inductively-coupled plasma mass spectrometer was applied to detect the source of trace elements based on their speciation profile along a 125-km stretch of a large river in the Canadian boreal forest. Both the concentration and proportion of bound trace elements were
increased by tributary inputs: bound As, Co, Fe, Mn, Pb, U, and Zn increased monotonically from upstream to downstream, increasingly resembling the speciation profile of tributaries. Principal component (PC) analysis also revealed tributary contributions of bound Cu, Ni, Th, V, and Y reflecting their higher concentrations in tributaries, and PC scores also increased monotonically from upstream–downstream. Monotonically decreasing concentrations of mainly ionic and small (i.e. <ca. 300 Da) As, Ba, Mo, and U species were also observed from upstream–downstream.
... Near the city of Manaus, the organic-rich Negro River ("black water", Sioli, 1968) meets the Andean sediment-rich Solimões River ("white water", Sioli, 1968). This confluence has been studied from the perspective of water dynamics (Laraque et al., 2009;Park and Latrubesse, 2015;Trevethan et al., 2015;Trevethan et al., 2016;Gualtieri et al., 2017;Ianniruberto et al., 2018) and of elemental behaviour across the mixing zone (Aucour et al., 2003;Benedetti et al., 2003;Moreira-Turcq et al., 2003;Guinoiseau et al., 2016a). Aucour et al. (2003) indicate that major dissolved elements (Al, Ca, K, Na, Mg) behave conservatively, except Fe and dissolved organic carbon (DOC), for which important losses are observed, as for particulate Al, Fe and organic carbon (POC). ...
... Aucour et al. (2003) indicate that major dissolved elements (Al, Ca, K, Na, Mg) behave conservatively, except Fe and dissolved organic carbon (DOC), for which important losses are observed, as for particulate Al, Fe and organic carbon (POC). Regarding the two trace metals targeted in the present study (Cu and Zn), Benedetti et al. (2003) report conservative behaviour of Cu based on the comparison of Cu fluxes in the unfiltered waters from the Negro River, Solimões River and Madeira River with those of the Amazon River at Óbidos. ...
The behaviour and fate of copper (Cu) and zinc (Zn) at river confluences is poorly understood, although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean. To identify and quantify the effect of such processes in river mixing zones, particulate Cu and Zn isotope signatures (δ⁶⁵CuSPM and δ⁶⁶ZnSPM) were measured along cross sections and vertical profiles at the largest river confluence in the world, the “Encontro das Aguas” mixing zone of the Amazon River, where the organic-rich Negro River meets the sediment-rich Solimões River. The Negro River suspended sediments, with highly variable Cu and Zn concentrations as well as δ⁶⁵CuSPM and δ⁶⁶ZnSPM, are mostly influenced by organic matter and by the lateritic cover of the watershed. The Solimões River suspended sediments, more homogeneous in Cu and Zn concentration and isotope composition across the river section, reflect the signature of weathered silicate-rich sediments derived from the Andes. The Solimões River supplies the majority of the suspended Cu and Zn to the Amazon River, and despite important flux losses across the confluence (−35% for Cu and −27% for Zn), δ⁶⁵CuSPM and δ⁶⁶ZnSPM show a conservative behaviour during the mixing. In the dissolved load, Cu concentrations and δ⁶⁵Cudiss, mostly supplied by the Solimões River, behave conservatively whereas Zn, derived mainly from the Negro River, suffers an important loss (−58%) that can be attributed to Zn adsorption onto the suspended sediments from the Solimões River. This transfer does not induce a significant δ⁶⁶ZnSPM shift in the Amazon River suspended sediments. Therefore, Cu and Zn isotope ratios in the suspended sediments behave conservatively through this confluence, which mixes two very chemically-contrasted rivers. Our findings thus suggest that the riverine isotopic information on the sources of particulate Cu and Zn is preserved during tributary mixing.
... The association of certain trace elements with colloidal particles may also play an important role in C-Q dynamics, especially in the Foreland-floodplain, where lowland tributaries are likely to supply abundant organic compounds [e.g., Stallard and Edmond, 1983;Benedetti et al., 2003]. While changes in element sizepartitioning between soluble and colloidal phases would not affect the bulk concentrations investigated here, colloidal particles may help remove elements from solution if they were to aggregate [e.g., Benedetti et al., 2003;Aucour et al., 2003;Bouchez et al., 2011;Mulholland et al., 2014] to form particles large enough to be caught in our filters (>0.2 lm). ...
... The association of certain trace elements with colloidal particles may also play an important role in C-Q dynamics, especially in the Foreland-floodplain, where lowland tributaries are likely to supply abundant organic compounds [e.g., Stallard and Edmond, 1983;Benedetti et al., 2003]. While changes in element sizepartitioning between soluble and colloidal phases would not affect the bulk concentrations investigated here, colloidal particles may help remove elements from solution if they were to aggregate [e.g., Benedetti et al., 2003;Aucour et al., 2003;Bouchez et al., 2011;Mulholland et al., 2014] to form particles large enough to be caught in our filters (>0.2 lm). However, in our data set, there does not appear to be a consistent pattern in trace element size-partitioning along the geomorphic gradient that could explain the near-universal change in trace element C-Q dynamics from Mountain-1 to Mountain-2 and further to the Foreland-floodplain. ...
Variations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute-generating reactions affect the dissolved load of rivers and streams. In this study, we investigate the concentration-runoff (C-Q) dynamics of a suite of major (Na, Mg, Ca, Si, K, and SO4) and trace (Al, Ba, Cd, Co, Cr, Cu, Fe, Ge, Li, Mn, Mo, Nd, Ni, Rb, Sr, U, V, and Zn) elements in nested catchments of variable size, spanning the geomorphic gradient from the Andes mountains to the Amazon foreland-floodplain. The major elements exhibit various degrees of dilution with increasing runoff at all sites, whereas the concentrations of most trace elements either increase or show no relationship with increasing runoff in the three larger catchments (160 to 28 000 km2 area). We show that the observed mainstem C-Q dynamics are influenced by variable mixing of tributaries with distinct C-Q relationships. Trace element C-Q relationships are more variable among tributaries relative to major elements, which could be the result of variations in geomorphology, lithology, and hydrology of the sub-catchments. Certain trace metals are also lost from solution during in-channel processes (possibly related to colloidal size-partitioning), which may exert an additional control on C-Q dynamics. Overall, we suggest that aggregation effects should be assessed in heterogeneous catchments before C-Q or ratio-Q relationships can be interpreted as reflecting catchment-wide solute generation processes and their relationship to hydrology. This article is protected by copyright. All rights reserved.
... Lakes, reservoirs, and floodplains act as important regulators on both the breakdown and storage of OM (Figure 1.8; Cole et al., 1994;Tranvik et al., 2009;Kellerman et al., 2015). Floodplains and lakes often represent areas of enhanced primary production, providing the river with fresh autochtonous OM (Junk et al., 1989;Melack and Forsberg, 2001;Benedetti et al., 2003). High levels of algal and aquatic plant production (Junk and Howard-Williams, 1984;Junk et al., 1989) can considerably enhance the overall export of DOM from a river system (Tockner et al., 1999;Hedges et al., 2000). ...
The purpose of this review is to highlight progress in unraveling carbon cycling dynamics across the continuum of landscapes, inland waters, coastal oceans, and the atmosphere. Earth systems are intimately interconnected, yet most biogeochemical studies focus on specific components in isolation. The movement of water drives the carbon cycle, and, as such, inland waters provide a critical intersection between terrestrial and marine biospheres. Inland, estuarine, and coastal waters are well studied in regions near centers of human population in the Northern hemisphere. However, many of the world's large river systems and their marine receiving waters remain poorly characterized, particularly in the tropics, which contribute to a disproportionately large fraction of the transformation of terrestrial organic matter to carbon dioxide, and the Arctic, where positive feedback mechanisms are likely to amplify global climate change. There are large gaps in current coverage of environmental observations along the aquatic continuum. For example, tidally-influenced reaches of major rivers and near-shore coastal regions around river plumes are often left out of carbon budgets due to a combination of methodological constraints and poor data coverage. We suggest that closing these gaps could potentially alter global estimates of CO2 outgassing from surface waters to the atmosphere by several-fold. Finally, in order to identify and constrain/embrace uncertainties in global carbon budget estimations it is important that we further adopt statistical and modeling approaches that have become well-established in the fields of oceanography and paleoclimatology, for example.
