Accessibility / Report Error

CO2 fluxes at the water-atmosphere interface in fluvial environments: an overview of studies in Brazilian rivers

Fluxos de CO2 na interface água - atmosfera em ambientes fluviais: um panorama sobre os estudos nos rios brasileiros

Abstract

Recent advances in Brazilian scientific production on CO2 fluxes at the water-atmosphere interface in rivers were reviewed, including estimates of CO2 partial pressure and fluxes. A total of 17 studies were reviewed. We compiled information regarding the location studied, the methodology used by each author, and the values of CO2 partial pressure (pCO2), CO2 fluxes (FCO2), and gas exchange coefficient (k) found in each region. The results were spatialized and synthesized. The important role of fluxes in CO2 degassing and their influence on climate change, as well as the global lack of data on these environments, were the main motivations for this study. The information was scarce, and most studies are focused on the Amazon Basin. However, high-resolution mapping of CO2 fluxes, at the scale of micro basins and streams, proved to be scarce. We emphasize, therefore, the importance of further studies in the country including other hydrographic regions, and in high resolution. These studies would add to our knowledge of how natural and anthropic processes influence CO2 flux, in addition to providing better estimates in tropical river systems.

Keywords:
climate change; CO2 emission; CO2 partial pressure

Resumo

Foram revisados os avanços recentes da produção científica brasileira sobre os fluxos de CO2 na interface água-atmosfera em ambientes fluviais, incluindo as estimativas de pressão parcial de CO2 e fluxos de CO2. Um total de 17 estudos foram revisados. Copilamos informações sobre a localização, a metodologia utilizada por cada autor e os valores da pressão parcial de CO2 (pCO2), fluxos de CO2 (FCO2) e coeficiente de troca gasosa (k) encontrados em cada região. Os resultados foram espacializados e sintetizados. O importante papel dos rios na desgasseificação de CO2 e sua influência nas mudanças climáticas, somado à escassez global de dados sobre esses ambientes, foram as principais motivações para a realização desta pesquisa. Foi possível demonstrar a baixa produtividade brasileira e a concentração de estudos na Bacia Amazônica. No entanto, mapeamentos dos fluxos de CO2 em alta resolução, em escala de microbacias e córregos, se mostraram escassos. Ressaltamos, portanto, a importância de mais estudos no país incluindo outras regiões hidrográficas, e em alta resolução. Estes estudos elucidariam o conhecimento de como os processos naturais e antrópicos influenciam no fluxo de CO2, além de melhores estimativas em sistemas fluviais tropicais.

Palavras-chave:
emissão de CO2; mudanças climáticas; pressão parcial de CO2

1. INTRODUCTION

The structure of the Planetary Boundaries (PB) (Rockström et al., 2009ROCKSTRÖM, J.; STEFFEN, W.; NOONE, K.; PERSSON, Å.; CHAPIN, F. S.; LAMBIN, E. F. et al. A safe operating space for humanity. Nature, v. 461, n. 7263, p. 472-475, 2009. https://doi.org/10.1038/461472a
https://doi.org/10.1038/461472a...
) has aroused great interest not only in science but also in politics and has been observed for years. Recent works have shown that climate change has already exceeded safe operation limits and entered a zone of uncertainty (increasing risks) (Persson et al., 2022PERSSON, L.; CARNEY ALMROTH, B. M.; COLLINS, C. D.; CORNELL, S.; DE WIT, C. A.; DIAMOND, M. L. et al. Outside the Safe Operating Space of the Planetary Boundary for Novel Entities. Environmental Science and Technology, v. 56, n. 3, p. 1510-1521, 2022. https://doi.org/10.1021/acs.est.1c04158
https://doi.org/10.1021/acs.est.1c04158...
; Steffen et al., 2015STEFFEN, W.; RICHARDSON, K.; ROCKSTRÖM, J.; CORNELL, S. E.; FETZER, I.; BENNETT, E. M. et al. Planetary boundaries: Guiding human development on a changing planet. Science, v. 347, n. 6223, 2015. https://doi.org/10.1126/science.1259855
https://doi.org/10.1126/science.1259855...
). Carbon dioxide (CO2) has been identified as one of the most important greenhouse gases responsible for global warming, and emission rates are increasing every year (IPCC, 2022IPCC. Climate Change 2022: Mitigation of Climate Change - Working Group III Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. 2022. 1991 p. Available at: Available at: https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf Access: 23 jan. 2023.
https://www.ipcc.ch/report/ar6/wg3/downl...
). Therefore, studies have been conducted worldwide to identify potential sources and sinks of CO2, improve flux estimates, and better understand climate change.

Aquatic ecosystems are important environments for CO2 gas exchange between the water surface and the atmosphere and play an important role in the carbon cycle (Keller et al., 2020KELLER, P. S.; CATALÁN, N.; VON SCHILLER, D.; GROSSART, H. P.; KOSCHORRECK, M.; OBRADOR, B. et al. Global CO2 emissions from dry inland waters share common drivers across ecosystems. Nature Communications, v. 11, n. 1, 2020. https://doi.org/10.1038/s41467-020-15929-y
https://doi.org/10.1038/s41467-020-15929...
). Global estimates suggest that river systems emit between 112 and 209 Tg C per month into the atmosphere (Liu et al., 2022LIU, S.; KUHN, C.; AMATULLI, G.; AHO, K.; BUTMAN, D. E.; ALLEN, G. H. et al. The importance of hydrology in routing terrestrial carbon to the atmosphere via global streams and rivers. Earth, Atmospheric, and Planetary Sciences, v. 119, n. 11, 2022. https://doi.org/10.1073/pnas.2106322119
https://doi.org/10.1073/pnas.2106322119...
). According to Gómez-Gener et al. (2021)GÓMEZ-GENER, L.; ROCHER-ROS, G.; BATTIN, T.; COHEN, M. J.; DALMAGRO, H. J.; DINSMORE, K. J. et al. Global carbon dioxide efflux from rivers enhanced by high nocturnal emissions. Nature Geoscience, v. 14, n. 5, p. 289-294, 2021. https://doi.org/10.1038/s41561-021-00722-3
https://doi.org/10.1038/s41561-021-00722...
, small rivers and streams are critical points in flux estimates in river environments. They account for about 85% of estimated CO2 emissions in inland waters, even though they represent less than 20% of the freshwater area. This occurs because, once there is a source of carbon input (whether natural or anthropogenic), the hydraulic parameters (such as depth and slope of the channel) and the dynamics of small rivers, influence and amplify the turbulence of the channel, intensifying the processes responsible for gas exchange (Horgby et al., 2019HORGBY, Å.; BOIX CANADELL, M.; ULSETH, A. J.; VENNEMANN, T. W.; BATTIN, T. J. High-Resolution Spatial Sampling Identifies Groundwater as Driver of CO2 Dynamics in an Alpine Stream Network. Journal of Geophysical Research: Biogeosciences, v. 124, n. 7, p. 1961-1976, 2019. https://doi.org/10.1029/2019JG005047
https://doi.org/10.1029/2019JG005047...
).

Land use and urban impacts on watersheds have also been studied as emission intensifiers in river environments, primarily due to high concentrations of organic matter (OM) (Cheng et al., 2020CHENG, G.; WANG, M.; CHEN, Y.; GAO, W. Source apportionment of water pollutants in the upstream of Yangtze River using APCS-MLR. Environmental Geochemistry and Health, v. 42, n. 11, p. 3795-3810, 2020. https://doi.org/10.1007/s10653-020-00641-z
https://doi.org/10.1007/s10653-020-00641...
; Herreid et al., 2021HERREID, A. M.; WYMORE, A. S.; VARNER, R. K.; POTTER, J. D.; MCDOWELL, W. H. Divergent Controls on Stream Greenhouse Gas Concentrations Across a Land-Use Gradient. Ecosystems, v. 24, n. 6, p. 1299-1316, 2021. https://doi.org/10.1007/s10021-020-00584-7
https://doi.org/10.1007/s10021-020-00584...
; Tang et al., 2023TANG, W.; XU, Y. J.; NI, M.; LI, S. Land use and hydrological factors control concentrations and diffusive fluxes of riverine dissolved carbon dioxide and methane in low-order streams. Water Research, v. 231, n. 119615, 2023. https://doi.org/10.1016/j.watres.2023.119615
https://doi.org/10.1016/j.watres.2023.11...
). Another hypothesis is that tropical regions contribute most of the freshwater CO2 fluxes (Wen et al., 2021WEN, Z.; SHANG, Y.; LYU, L.; LI, S.; TAO, H.; SONG, K. A review of quantifying pco2 in inland waters with a global perspective: Challenges and prospects of implementing remote sensing technology. Remote Sensing, v. 13, n. 13, 2021. https://doi.org/10.3390/rs13234916
https://doi.org/10.3390/rs13234916...
). Liu et al. (2022)LIU, S.; KUHN, C.; AMATULLI, G.; AHO, K.; BUTMAN, D. E.; ALLEN, G. H. et al. The importance of hydrology in routing terrestrial carbon to the atmosphere via global streams and rivers. Earth, Atmospheric, and Planetary Sciences, v. 119, n. 11, 2022. https://doi.org/10.1073/pnas.2106322119
https://doi.org/10.1073/pnas.2106322119...
, show that tropical rivers have higher CO2 fluxes (3,220 g C m-2 y-1) than Arctic (1,750 g C m-2 y-1) and temperate (2,280 g C m-2 y-1) rivers. Thus, rivers have become extremely relevant ecosystems, specifically with respect to CO2 fluxes and their contribution to climate change. However, estimates of CO2 fluxes still have great uncertainties (Zhang et al., 2020ZHANG, T.; LI, J.; PU, J.; MARTIN, J. B.; WANG, S.; YUAN, D. Rainfall possibly disturbs the diurnal pattern of CO2 degassing in the Lijiang River, SW China. Journal of Hydrology, v. 590, 2020. https://doi.org/10.1016/j.jhydrol.2020.125540
https://doi.org/10.1016/j.jhydrol.2020.1...
). Thus, recently, rivers are being seen as important ecosystems in CO2 fluxes, although their estimates still have large uncertainties (Zhang et al., 2020ZHANG, T.; LI, J.; PU, J.; MARTIN, J. B.; WANG, S.; YUAN, D. Rainfall possibly disturbs the diurnal pattern of CO2 degassing in the Lijiang River, SW China. Journal of Hydrology, v. 590, 2020. https://doi.org/10.1016/j.jhydrol.2020.125540
https://doi.org/10.1016/j.jhydrol.2020.1...
). In addition, studies on spatial and temporal variations, mainly in small river basins, have gained prominence since CO2 emissions vary considerably in space and time due to hydrodynamics, which strongly influences biogeochemistry and seasonal variations (Barefoot et al., 2019BAREFOOT, E.; PAVELSKY, T. M.; ALLEN, G. H.; ZIMMER, M. A.; MCGLYNN, B. L. Temporally variable stream width and surface area distributions in a headwater Catchment. Water Resources Research, v. 55, n. 8, p. 7166-7181, 2019. https://doi.org/10.1029/2018WR023877
https://doi.org/10.1029/2018WR023877...
; Zimmer and McGlynn, 2018ZIMMER, M. A.; MCGLYNN, B. L. Lateral, Vertical, and Longitudinal Source Area Connectivity Drive Runoff and Carbon Export Across Watershed Scales. Water Resources Research, v. 54, n. 3, p. 1576-1598, 2018. https://doi.org/10.1002/2017WR021718
https://doi.org/10.1002/2017WR021718...
).

Despite the great importance, studies of CO2 fluxes at the water-atmosphere interface, focusing on river environments, are still rare and sparse in Brazil. This is partly due to the cost of analyses and the lack of research infrastructure and support, common in developing countries. However, a critical knowledge gap, especially in the Amazon River Basin, remains the importance of rivers and streams as atmospheric sources of CO2. According to Richey et al. (2002)RICHEY, J. E.; MELACK, J. M.; AUFDENKAMPE, A. K.; BALLESTER, V. M.; HESS, L. L. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, v. 416, n. 6881, p. 617-620, 2002. https://doi.org/10.1038/416617a
https://doi.org/10.1038/416617a...
, although the Amazon River systems have high fluxes, they may not significantly contribute to the global carbon balance since estimates suggest that their CO2 emissions are nearly equal to the amount of carbon sequestered. In this manner, the overall balance tends to nullify.

This study demonstrates the recent status of Brazilian scientific production on CO2 fluxes in river systems. The main methods currently used were evaluated and discussed, and the variables related to degassing were synthesized to provide estimations of the CO2 fluxes found in Brazilian rivers. From the analysis of the studies found, knowledge gaps on the subject in the Brazilian territory were identified.

2. MATERIAL AND METHODS

Table 1 shows the workflow of this study. The literature review was based on the methodology described by Clark et al. (2020)CLARK, J.; GLASZIOU, P.; DEL MAR, C.; BANNACH-BROWN, A.; STEHLIK, P.; SCOTT, A. M. A full systematic review was completed in 2 weeks using automation tools: a case study. Journal of Clinical Epidemiology, v. 121, p. 81-90, 2020. https://doi.org/10.1016/j.jclinepi.2020.01.008
https://doi.org/10.1016/j.jclinepi.2020....
. The research was conducted on the Scopus, Scielo, and Periódicos CAPES platforms. The keywords used in the bibliographic search were defined using tools such as Systematic Review Accelerator (SRA) using Word Frequency Analyzer and VOSviewer. The most frequently found words were “dissolved inorganic carbon”, “carbon dioxide”, “CO2 partial pressure”, “CO2 emission” and “CO2 flow”. These keywords were used together with the transversal word “water” and the Boolean operator “AND”. At the end of the searches, a total of 803 studies were cataloged.

Shared citations between research platforms (duplicates) were checked and removed using Mendeley and SRA - Duplicator. Since the objective of the research is to demonstrate the recent status of scientific advances in Brazil, a period of approximately 20 years was defined for analysis. Therefore, studies written before 2000 and research not conducted in Brazilian aquatic environments (uncontained), were also excluded using the SRA-Helper and Mendeley tools. After filtering, the database consisted of 105 studies published between 2000 and 2022 in aquatic environments on Brazilian territory.

The publications included in the database were classified into different types of aquatic environments based on the title, abstract, and keyword, such as coastal-marine environments (28 studies), reservoirs (26 studies), lagoon environments (27 studies), and river environments (24 studies). Only the river environments were evaluated and reviewed in the study. Of the 24 studies of the river, only 17 contained pCO2 and/or FCO2, which were analyzed (Table S1 in Supplementary Material).

Table 1.
Summary of workflow and results obtained. * Total of deleted files = duplicates + uncontained. SRA = Systematic Review Accelerator.

3. RESULTS AND DISCUSSION

3.1. Bibliographic research

Figure 1 shows the spatial distribution of the study and the number of publications per year. In the last 20 years, the number of annual publications on CO2 fluxes in the river has not exceeded four articles per year, which explains the deficit of studies in river environments in Brazil. According to the research conducted in this work, the majority of the 12 Brazilian Hydrographic Regions have not presented any data on CO2 fluxes. Of the works found, most were in the Amazon Basin, mainly in the central region of the basin in the state of Amazon (AM).

Figure 1.
Distribution of research and publications number over the last 20 years in Brazil. a) hydrographic regions of Brazil with the location of the studies found and b) number of publications over the years.

3.2. Analytical methods

Different techniques are available for both direct and indirect measurements of pCO2 and FCO2. Currently, there is no consensus on the different methods used in river environments. Table 2 shows the methods used in the analyzed studies.

The partial pressure of CO2 corresponds to the saturation degree between a water sample and gaseous CO2 (Dickson, 2010DICKSON, A. G. The carbon dioxide system in seawater: equilibrium chemistry and measurements. In: GATTUSO, Jean-Pierre et al. Guide to best practices for ocean acidification research and data reporting. Publications Office of the European Union, 2010.). Direct methods involve two analytical steps: (i) headspace and (ii) gas-phase analysis. The headspace technique concerns the equilibrium relationship between the liquid and gas phases, which in turn follows Henry's Law. The equilibrium can be obtained by an equilibrator (without limitation of water volume) or even by glass vials or syringes (with limitation of water volume). After the equilibration time, the gaseous phase is measured in a gas analyzer. Indirect measurements, in turn, are performed from the thermodynamic equilibrium using the dissolution constants and at least some variables that make up the carbonate system, such as pH, total alkalinity (TA), and dissolved inorganic carbono (DIC).

Table 2.
Description and comparison of the methods used. pCO2 = CO2 partial pressure and FCO2 = CO2 fluxes at the water-atmosphere interface. * = value obtained with floating chambers ** = average value obtained for the Amazon with floating chambers; - no data and a = not specified.

One of the most important studies discussing indirect pCO2 is that of Abril et al. (2015)ABRIL, G.; BOUILLON, S.; DARCHAMBEAU, F.; TEODORU, C. R.; MARWICK, T. R.; TAMOOH, F. et al. Technical note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters. Biogeosciences, v. 12, n. 1, p. 67-78, 2015. https://doi.org/10.5194/bg-12-67-2015
https://doi.org/10.5194/bg-12-67-2015...
, in which it was shown that environments with low pH (<6.0) and high concentrations of organic matter can interfere with modeling from the carbonate system and produce an overestimation of pCO2 and consequently atmospheric emissions. On the other hand, headspace techniques are sometimes costly, and difficult-to-access environments can make studies using this methodology infeasible. Moreover, it is still necessary to use the equipment to measure gas samples. Therefore, indirect calculation of pCO2 is still considered a useful tool for flux estimation and is used and accepted by several authors, since its prudent use and understanding of its limitations allows the assessment of the orders of magnitude in which fluxes fit.

The magnitude and direction of CO2 fluxes are proportional to the difference in CO2 gas concentration between the water-atmosphere interface and the gas exchange coefficient (k). Direct flux measurements can be made using, for example, (i) a floating chamber (Frankignoulle et al., 1998FRANKIGNOULLE, M.; ABRIL, G.; BORGES, A.; BOURGE, I.; CANON, C.; DELILLE, B. et al. Carbon Dioxide Emission from European Estuaries. Science, v. 282, n. 5388, p. 434-436, 1998. https://doi.org/10.1126/science.282.5388.434
https://doi.org/10.1126/science.282.5388...
); (ii) the correlation of turbulent eddies (McGillis et al., 2001MCGILLIS, W. R.; EDSON, J. B.; HARE, J. E.; FAIRALL, C. W. Direct covariance air-sea CO 2 fluxes. Journal of Geophysical Research: Oceans, v. 106, n. C8, p. 16729-16745, 2001. https://doi.org/10.1029/2000JC000506
https://doi.org/10.1029/2000JC000506...
); (iii) the flow gradient (Zappa et al., 2003ZAPPA, C. J.; RAYMOND, P. A.; TERRAY, E. A.; MCGILLIS, W. R. Variation in Surface Turbulence and the Gas Transfer Velocity over a Tidal Cycle in a Macro-tidal Estuary. Estuaries, v. 26, n. 6, 2003. https://doi.org/10.1007/BF02803649
https://doi.org/10.1007/BF02803649...
) or (iv) volatile tracers (Raymond et al., 2012RAYMOND, P. A.; ZAPPA, C. J.; BUTMAN, D.; BOTT, T. L.; POTTER, J.; MULHOLLAND, P. et al. Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, v. 2, n. 1, p. 41-53, 2012. https://doi.org/10.1215/21573689-1597669
https://doi.org/10.1215/21573689-1597669...
; Richey et al., 2002RICHEY, J. E.; MELACK, J. M.; AUFDENKAMPE, A. K.; BALLESTER, V. M.; HESS, L. L. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, v. 416, n. 6881, p. 617-620, 2002. https://doi.org/10.1038/416617a
https://doi.org/10.1038/416617a...
). The correlation of turbulent eddies and the flow gradient methods are more appropriate and used for coastal and marine environments, not being methodologies commonly used in river systems. The floating chamber has been most commonly used in river systems due to its low cost, ability to measure in a short time, and simplicity. The main criticisms and discussions of this method are the possible interference of the floating chamber in surface turbulence and consequently in the values of k (Kokic et al., 2018KOKIC, J.; SAHLÉE, E.; SOBEK, S.; VACHON, D.; WALLIN, M. High spatial variability of gas transfer velocity in streams revealed by turbulence measurements. Inland Waters, 2018. https://dx.doi.org/10.1080/20442041.2018.1500228
https://dx.doi.org/10.1080/20442041.2018...
; Lorke et al., 2015LORKE, A.; BODMER, P.; NOSS, C.; ALSHBOUL, Z.; KOSCHORRECK, M.; SOMLAI-HAASE, C. et al. Technical note: drifting versus anchored flux chambers for measuring greenhouse gas emissions from running waters. Biogeosciences, v. 12, p. 7013-7024, 2015. https://doi.org/10.5194/bg-12-7013-2015
https://doi.org/10.5194/bg-12-7013-2015...
; Raymond and Cole, 2001RAYMOND, P. A.; COLE, J. J. Technical Notes and Comments Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity. Estuaries, v. 24, n. 2, 2001. ). However, Kremer et al. (2003)KREMER, J. N.; NIXON, S. W.; BUCKLEY, B.; ROQUES, P. Technical Note: Conditions for Using the Floating Chamber Method to Estimate Air-Water Gas Exchange. Estuaries, v. 26, n. 4A, 2003., obtained consistent values using this method. In Brazil, this technique is widely used.

The indirect method of FCO2 is based on the theoretical diffusion model, which is based on the difference between CO2 concentrations in water and air and the gas exchange coefficient (k) (Liss and Slater, 1974LISS, P.; SLATER, P. Flux of Gases across the Air-Sea Interface. Nature, v. 247, p. 181-184, 1974. https://doi.org/10.1038/247181a0
https://doi.org/10.1038/247181a0 ...
). The gas exchange coefficient (k) corresponds to the gas transfer rate and can be determined using empirical equations. Currently, there is a debate about the importance of a good measurement of k in rivers. A common application is to parameterize k to k600 by normalizing with the Schmidt number (Sc), which, in turn, corresponds to the ratio between the viscosity of the liquid and the diffusion constant of the CO2 gas at a given temperature and salinity (Jähne et al., 1987JÄHNE, B. et al. On the parameters influencing air-water gas exchange. Journal of Geophysical Research, v. 92, n. C2, p. 1937-1949, 1987. https://doi.org/10.1029/JC092iC02p01937
https://doi.org/10.1029/JC092iC02p01937...
). The Schmidt number for carbon dioxide (ScCO2) in freshwater and at a temperature of 20°C is equal to 600 (ScCO2 (20°C) = 600). Different equations for calculating k in fluvial environments are discussed in the literature (Raymond et al., 2012RAYMOND, P. A.; ZAPPA, C. J.; BUTMAN, D.; BOTT, T. L.; POTTER, J.; MULHOLLAND, P. et al. Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, v. 2, n. 1, p. 41-53, 2012. https://doi.org/10.1215/21573689-1597669
https://doi.org/10.1215/21573689-1597669...
).

In rivers, k values are strongly influenced by water turbulence, which in turn can be influenced by flow rate and flow velocity, channel slope as well as friction generated on the surface due to wind speed, for example (Butman and Raymond, 2011BUTMAN, D.; RAYMOND, P. Significant efflux of carbon dioxide from streams and rivers in the United States. Nature Geoscience, v. 4, p. 839-842, 2011. https://doi.org/10.1038/ngeo1294
https://doi.org/10.1038/ngeo1294...
). In a large hydrographic watershed, the wind has a large influence on the value of k. However, in small rivers and streams, wind velocity becomes less important and can be neglected, and the effect of water friction on the river bottom and current velocity predominate in the values of k (Alin et al., 2011ALIN, S. R.; RASERA, M. D. F. F. L.; SALIMON, C. I.; RICHEY, J. E.; HOLTGRIEVE, G. W.; KRUSCHE, A. V. et al. Physical controls on carbon dioxide transfer velocity and flux in low-gradient river systems and implications for regional carbon budgets. Journal of Geophysical Research: Biogeosciences, v. 116, n. 1, 2011. https://doi.org/10.1029/2010JG001398
https://doi.org/10.1029/2010JG001398...
). Thus, the hydraulic and geomorphological parameters become more important. The shallower the depth of a river, the greater the friction from the bottom, and the greater the values of k (Raymond et al., 2012RAYMOND, P. A.; ZAPPA, C. J.; BUTMAN, D.; BOTT, T. L.; POTTER, J.; MULHOLLAND, P. et al. Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, v. 2, n. 1, p. 41-53, 2012. https://doi.org/10.1215/21573689-1597669
https://doi.org/10.1215/21573689-1597669...
). The slope of the channel also becomes a relevant factor because the greater the slope, the greater the flow velocity, which increases the values of k.

Several equations for the calculation of k600 can be found in the literature. Table 3 shows a compilation of the main equations currently found in the literature. Among the studies using k600, the equation by Alin et al. (2011)ALIN, S. R.; RASERA, M. D. F. F. L.; SALIMON, C. I.; RICHEY, J. E.; HOLTGRIEVE, G. W.; KRUSCHE, A. V. et al. Physical controls on carbon dioxide transfer velocity and flux in low-gradient river systems and implications for regional carbon budgets. Journal of Geophysical Research: Biogeosciences, v. 116, n. 1, 2011. https://doi.org/10.1029/2010JG001398
https://doi.org/10.1029/2010JG001398...
based on wind velocity was the most used, followed by the equations by Raymond et al. (2012)RAYMOND, P. A.; ZAPPA, C. J.; BUTMAN, D.; BOTT, T. L.; POTTER, J.; MULHOLLAND, P. et al. Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, v. 2, n. 1, p. 41-53, 2012. https://doi.org/10.1215/21573689-1597669
https://doi.org/10.1215/21573689-1597669...
using slope and flow velocity. Studies comparing the values of k600 using different methods are extremely scarce, although there is evidence that the hydraulic parameters are extremely important factors in the correct estimation of k600, as shown by Raymond et al. (2012)RAYMOND, P. A.; ZAPPA, C. J.; BUTMAN, D.; BOTT, T. L.; POTTER, J.; MULHOLLAND, P. et al. Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, v. 2, n. 1, p. 41-53, 2012. https://doi.org/10.1215/21573689-1597669
https://doi.org/10.1215/21573689-1597669...
. Another important discussion is the fact that many studies use static values of k600 for the entire watershed. Alin et al. (2011)ALIN, S. R.; RASERA, M. D. F. F. L.; SALIMON, C. I.; RICHEY, J. E.; HOLTGRIEVE, G. W.; KRUSCHE, A. V. et al. Physical controls on carbon dioxide transfer velocity and flux in low-gradient river systems and implications for regional carbon budgets. Journal of Geophysical Research: Biogeosciences, v. 116, n. 1, 2011. https://doi.org/10.1029/2010JG001398
https://doi.org/10.1029/2010JG001398...
conclude in their study on the Amazon River that small rivers have a large variability in the value of k600, highlighting the importance of hydraulic parameters. Ulseth et al. (2019)ULSETH, A. J.; HALL, R. O.; BOIX CANADELL, M.; MADINGER, H. L.; NIAYIFAR, A.; BATTIN, T. J. Distinct air-water gas exchange regimes in low- and high-energy streams. Nature Geoscience, v. 12, n. 4, p. 259-263, 2019. https://doi.org/10.1038/s41561-019-0324-8
https://doi.org/10.1038/s41561-019-0324-...
demonstrate the relevance of slope in small rivers, especially in mountainous areas, and conclude that increasing slope increases the rate of energy dissipation, which in turn increases flow. Thus, many rivers may have been underestimated by using a k600 for the entire length of the watershed. Therefore, the importance of further investigation of the k600 and comparative studies on the methods is emphasized.

Table 3.
Main k600 measurement methods. Average wind speed at 10 m above the surface (ū10, m s-1); water current velocity (V, m s-1), water depth (D, m), discharge (Q, m3 s-1), stream slope (S, m m-1), Froude number ( Fr = V⁄(gD)0,5) and energy dissipation rate (eD = gSv, m2 s-3 where g = acceleration due to gravity (9.81 m s-1); aeD < 0.02 and beD > 0.02).

3.3. CO2 fluxes in rivers in Brazilian territory

Table 4 summarizes the values reported in the Brazilian area. Many studies have shown that rivers are environments that are supersaturated with CO2 compared to the atmosphere (Wen et al., 2021WEN, Z.; SHANG, Y.; LYU, L.; LI, S.; TAO, H.; SONG, K. A review of quantifying pco2 in inland waters with a global perspective: Challenges and prospects of implementing remote sensing technology. Remote Sensing, v. 13, n. 13, 2021. https://doi.org/10.3390/rs13234916
https://doi.org/10.3390/rs13234916...
). In the studies examined, the variation of pCO2 ranged from 141 - 26,975 µatm. Despite methodological differences, flow velocity was cited as one of the most important factors influencing the magnitude, with high flow episodes generally showing higher pCO2. Channel slope (Sorribas et al., 2017SORRIBAS, M. V.; DA MOTTA MARQUES, D.; CASTRO, N. M. DOS R.; FAN, F. M. Fluvial carbon export and CO2 efflux in representative nested headwater catchments of the eastern La Plata River Basin. Hydrological Processes, v. 31, n. 5, p. 995-1006, 2017. https://doi.org/10.1002/hyp.11076
https://doi.org/10.1002/hyp.11076...
), lithology (Machado et al., 2022MACHADO, D. V.; ALMEIDA, G. S.; MARQUES, E. D.; SILVA-FILHO, E. V. Carbon fluxes in a carbonate rock dominated micro basin of the Quadrilátero Ferrífero, Brazil. Environmental Science and Pollution Research, v. 29, n. 50, p. 76177-76191, 2022. https://doi.org/10.1007/s11356-022-21155-4
https://doi.org/10.1007/s11356-022-21155...
), vegetation, flooded areas, DO concentration, and water temperature (Amaral et al., 2019AMARAL, J. H. F.; FARJALLA, V. F.; MELACK, J. M.; KASPER, D.; SCOFIELD, V.; BARBOSA, P. M. et al. Seasonal and spatial variability of CO2 in aquatic environments of the central lowland Amazon basin. Biogeochemistry, v. 143, n. 1, p. 133-149, 2019. https://doi.org/10.1007/s10533-019-00554-9
https://doi.org/10.1007/s10533-019-00554...
) were also identified as important factors influencing pCO2. The anthropogenic impact was studied by (Andrade et al., 2011ANDRADE, T. M. B.; CAMARGO, P. B.; SILVA, D. M. L.; PICCOLO, M. C.; VIEIRA, S. A.; ALVES, L. F. et al. Dynamics of dissolved forms of carbon and inorganic nitrogen in small watersheds of the coastal Atlantic forest in southeast Brazil. Water, Air, and Soil Pollution, v. 214, n. 1-4, p. 393-408, 2011. https://doi.org/10.1007/s11270-010-0431-z
https://doi.org/10.1007/s11270-010-0431-...
; Lopes Da Silva et al., 2007LOPES DA SILVA, D. M.; OMETTO, J. P. H. B.; LOBO, G. A.; LIMA, W. P.; SCARANELLO, M. A. et al. Can Land Use Changes Alter Carbon, Nitrogen and Major Ion Transport In Subtropical Brazilian Streams? Scientia Agricola, v. 64, n. 4, 2007. https://doi.org/10.1590/S0103-90162007000400002
https://doi.org/10.1590/S0103-9016200700...
), and they found that rivers that drain urban areas have high values compared to non-urban areas, showing the impact of organic matter as an influencing factor on the increase of pCO2. The high variability in pCO2 values supports the hypothesis that in fluvial environments, hydro-geomorphological changes along the river channel lead to changes in pCO2 values, supporting the idea that general estimates for a large watershed may lead to underestimates of pCO2 given the importance of small rivers and streams (Ward et al., 2017WARD, N. D.; BIANCHI, T. S.; MEDEIROS, P. M.; SEIDEL, M.; RICHEY, J. E.; KEIL, R. G. et al. Where carbon goes when water flows: Carbon cycling across the aquatic continuum. Frontiers in Marine Science, v. 4, 2017. https://doi.org/10.3389/fmars.2017.00007
https://doi.org/10.3389/fmars.2017.00007...
).

Table 4.
Values comparison found in studies. pCO2 = CO2 partial pressure and FCO2 = CO2 fluxes at the water-atmosphere interface. - no data. a = 210 T C y-1; b = 641 - 12,553 (mg m-2 d-1); c = 0.44 g C m-2 y-1; d = 1.2 Mg C ha-1 y-1 (470 Tg C y-1).

The estimated global variation of k600 in the river is calculated to be 8-33 cm h-1, and for large tropical and temperate rivers, it is 5-31 cm h-1 (Li et al., 2019LI, S.; MAO, R.; MA, Y.; SARMA, V. V. S. S. Gas transfer velocities of CO2 in subtropical monsoonal climate streams and small rivers. Biogeosciences, v. 16, n. 3, p. 681-693, 2019. https://doi.org/10.5194/bg-16-681-2019
https://doi.org/10.5194/bg-16-681-2019...
). The values reported in the studies were extremely wide, sometimes lower or higher than the global estimates. The magnitude of k600 has generated much debate, but it is known that this parameter is influenced by several physical variables, which increases its variation along the channel. Hilly terrain, high flow velocities, channel bottom roughness, and water depth are extremely influential factors on k600 values, especially in small rivers and streams where shallow water increases turbulence due to high channel bottom shear. On this basis, Ward et al. (2017)WARD, N. D.; BIANCHI, T. S.; MEDEIROS, P. M.; SEIDEL, M.; RICHEY, J. E.; KEIL, R. G. et al. Where carbon goes when water flows: Carbon cycling across the aquatic continuum. Frontiers in Marine Science, v. 4, 2017. https://doi.org/10.3389/fmars.2017.00007
https://doi.org/10.3389/fmars.2017.00007...
report that the probability that current global CO2 flux budgets are underestimated due to a lack of budgets and a poor understaning of the role of small rivers is due to the high variability of k600 in smaller rivers (< 100 meters).

In the Amazon Basin, where most of the studies dealt with in Brazil, not all studies have considered k600 variations along the river channel. In 2013, studies in the central region of the basin included small rivers in their estimates (Rasera et al., 2013RASERA, M. DE F. F. L.; KRUSCHE, A. V.; RICHEY, J. E.; BALLESTER, M. V. R.; VICTÓRIA, R. L. Spatial and temporal variability of pCO2 and CO2 efflux in seven Amazonian Rivers. Biogeochemistry, n. 116, n. 1-3, p. 241-259, 2013. https://doi.org/10.1007/s10533-013-9854-0
https://doi.org/10.1007/s10533-013-9854-...
). Another factor in the Amazon Basin is the inclusion of the lower Amazon River, which according to Sawakuchi et al. (2017)SAWAKUCHI, H. O.; NEU, V.; WARD, N. D.; BARROS, M. DE L. C.; VALERIO, A. M.; GAGNE-MAYNARD, W. et al. Carbon Dioxide Emissions along the Lower Amazon River. Frontiers in Marine>Science, v. 4, 2017. https://doi.org/10.3389/fmars.2017.00076
https://doi.org/10.3389/fmars.2017.00076...
, had higher fluxes than previous estimates for the entire basin (Richey et al., 2002RICHEY, J. E.; MELACK, J. M.; AUFDENKAMPE, A. K.; BALLESTER, V. M.; HESS, L. L. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, v. 416, n. 6881, p. 617-620, 2002. https://doi.org/10.1038/416617a
https://doi.org/10.1038/416617a...
).

For the other regions of the country, there are isolated and specific studies that make a broader and deeper interpretation impossible and show the existing gap in Brazil. However, the common fact observed in this review is the high pCO2 values, indicating C supersaturation in the observed fluvial environments. It is worth emphasizing the importance of small and large scales studies, including temporal assessment to visualize the flux variations with hydrography, and spatial assessment to obtain a deeper understanding of the variables affecting the dynamics of CO2 fluxes.

4. CONCLUSIONS

The literature search showed a scarcity of studies on CO2 fluxes along the numerous hydrographic basins of the Brazilian territory, highlighting the need for further studies, especially in small rivers and streams. The analysed river systems exhibited significant variation in FCO2. Among the studies utilizing the theoretical diffusion model, a scant number employed hydraulic parameters to calculate the k600, with only two studies employing different equations in their estimations. Direct methods for measuring fluxes, the floating chamber were widely utilized. Few studies were identified concerning small rivers and streams, and in some cases, solely reporting pCO2 values.

Although most studies are conducted in the Amazon River Basin, this does not mean that there is a large number of research works in the region. The Amazon Basin is known worldwide, and knowledge and accurate estimation of its fluxes are extremely important in the context of global climate change. However, the results presented in this study show that knowledge and estimates of fluxes in other regions of the country need to be expanded. In addition, due to the variability of k values along the channel, the importance of high-resolution studies, on a scale of small rivers and streams is emphasized to better understand how natural and anthropic factors, such as different biomes, zones climatic conditions and changes in land use, can influence the behavior of CO2 fluxes.

Thus, based on this bibliographic survey, two major gaps in knowledge about CO2 fluxes in Brazilian river systems are highlighted (i) the carrying out of studies that cover other Brazilian basins besides the Amazon Basin and (ii) the inclusion of an estimation of fluxes using high-resolution mapping, that is, on a scale of small and micro basins. This study aimed to contribute not only to the identification of these gaps but also to highlight the importance of small rivers and streams in the dynamics of CO2 fluxes in continental environments.

5. ACKNOWLEDGEMENTS

This study was funded by institutions Carlos Chagas Foundation Son of Research Support of the State of Rio de Janeiro (FAPERJ) - E-26/200.695/2021 (262375).

6. REFERENCES

  • ABRIL, G.; BOUILLON, S.; DARCHAMBEAU, F.; TEODORU, C. R.; MARWICK, T. R.; TAMOOH, F. et al Technical note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters. Biogeosciences, v. 12, n. 1, p. 67-78, 2015. https://doi.org/10.5194/bg-12-67-2015
    » https://doi.org/10.5194/bg-12-67-2015
  • ABRIL, G.; MARTINEZ, J.-M.; ARTIGAS, L. F.; MOREIRA-TURCQ, P.; BENEDETTI, M. F.; VIDAL, L. et al Amazon River carbon dioxide outgassing fuelled by wetlands. Nature, v. 505, n. 7483, p. 395-398, 2014. https://doi.org/10.1038/nature12797
    » https://doi.org/10.1038/nature12797
  • ALIN, S. R.; RASERA, M. D. F. F. L.; SALIMON, C. I.; RICHEY, J. E.; HOLTGRIEVE, G. W.; KRUSCHE, A. V. et al Physical controls on carbon dioxide transfer velocity and flux in low-gradient river systems and implications for regional carbon budgets. Journal of Geophysical Research: Biogeosciences, v. 116, n. 1, 2011. https://doi.org/10.1029/2010JG001398
    » https://doi.org/10.1029/2010JG001398
  • ALMEIDA, R. M.; PACHECO, F. S.; BARROS, N.; ROSI, E.; ROLAND, F. Extreme floods increase CO2 outgassing from a large Amazonian river. Limnology and Oceanography, v. 62, n. 3, p. 989-999, 2017. https://doi.org/10.1002/lno.10480
    » https://doi.org/10.1002/lno.10480
  • AMARAL, J. H. F.; FARJALLA, V. F.; MELACK, J. M.; KASPER, D.; SCOFIELD, V.; BARBOSA, P. M. et al Seasonal and spatial variability of CO2 in aquatic environments of the central lowland Amazon basin. Biogeochemistry, v. 143, n. 1, p. 133-149, 2019. https://doi.org/10.1007/s10533-019-00554-9
    » https://doi.org/10.1007/s10533-019-00554-9
  • ANDRADE, T. M. B.; CAMARGO, P. B.; SILVA, D. M. L.; PICCOLO, M. C.; VIEIRA, S. A.; ALVES, L. F. et al Dynamics of dissolved forms of carbon and inorganic nitrogen in small watersheds of the coastal Atlantic forest in southeast Brazil. Water, Air, and Soil Pollution, v. 214, n. 1-4, p. 393-408, 2011. https://doi.org/10.1007/s11270-010-0431-z
    » https://doi.org/10.1007/s11270-010-0431-z
  • BAREFOOT, E.; PAVELSKY, T. M.; ALLEN, G. H.; ZIMMER, M. A.; MCGLYNN, B. L. Temporally variable stream width and surface area distributions in a headwater Catchment. Water Resources Research, v. 55, n. 8, p. 7166-7181, 2019. https://doi.org/10.1029/2018WR023877
    » https://doi.org/10.1029/2018WR023877
  • BUTMAN, D.; RAYMOND, P. Significant efflux of carbon dioxide from streams and rivers in the United States. Nature Geoscience, v. 4, p. 839-842, 2011. https://doi.org/10.1038/ngeo1294
    » https://doi.org/10.1038/ngeo1294
  • CHENG, G.; WANG, M.; CHEN, Y.; GAO, W. Source apportionment of water pollutants in the upstream of Yangtze River using APCS-MLR. Environmental Geochemistry and Health, v. 42, n. 11, p. 3795-3810, 2020. https://doi.org/10.1007/s10653-020-00641-z
    » https://doi.org/10.1007/s10653-020-00641-z
  • CLARK, J.; GLASZIOU, P.; DEL MAR, C.; BANNACH-BROWN, A.; STEHLIK, P.; SCOTT, A. M. A full systematic review was completed in 2 weeks using automation tools: a case study. Journal of Clinical Epidemiology, v. 121, p. 81-90, 2020. https://doi.org/10.1016/j.jclinepi.2020.01.008
    » https://doi.org/10.1016/j.jclinepi.2020.01.008
  • DICKSON, A. G. The carbon dioxide system in seawater: equilibrium chemistry and measurements. In: GATTUSO, Jean-Pierre et al Guide to best practices for ocean acidification research and data reporting. Publications Office of the European Union, 2010.
  • FRANKIGNOULLE, M.; ABRIL, G.; BORGES, A.; BOURGE, I.; CANON, C.; DELILLE, B. et al Carbon Dioxide Emission from European Estuaries. Science, v. 282, n. 5388, p. 434-436, 1998. https://doi.org/10.1126/science.282.5388.434
    » https://doi.org/10.1126/science.282.5388.434
  • GÓMEZ-GENER, L.; ROCHER-ROS, G.; BATTIN, T.; COHEN, M. J.; DALMAGRO, H. J.; DINSMORE, K. J. et al Global carbon dioxide efflux from rivers enhanced by high nocturnal emissions. Nature Geoscience, v. 14, n. 5, p. 289-294, 2021. https://doi.org/10.1038/s41561-021-00722-3
    » https://doi.org/10.1038/s41561-021-00722-3
  • HERREID, A. M.; WYMORE, A. S.; VARNER, R. K.; POTTER, J. D.; MCDOWELL, W. H. Divergent Controls on Stream Greenhouse Gas Concentrations Across a Land-Use Gradient. Ecosystems, v. 24, n. 6, p. 1299-1316, 2021. https://doi.org/10.1007/s10021-020-00584-7
    » https://doi.org/10.1007/s10021-020-00584-7
  • HORGBY, Å.; BOIX CANADELL, M.; ULSETH, A. J.; VENNEMANN, T. W.; BATTIN, T. J. High-Resolution Spatial Sampling Identifies Groundwater as Driver of CO2 Dynamics in an Alpine Stream Network. Journal of Geophysical Research: Biogeosciences, v. 124, n. 7, p. 1961-1976, 2019. https://doi.org/10.1029/2019JG005047
    » https://doi.org/10.1029/2019JG005047
  • IPCC. Climate Change 2022: Mitigation of Climate Change - Working Group III Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. 2022. 1991 p. Available at: Available at: https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf Access: 23 jan. 2023.
    » https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf
  • JÄHNE, B. et al On the parameters influencing air-water gas exchange. Journal of Geophysical Research, v. 92, n. C2, p. 1937-1949, 1987. https://doi.org/10.1029/JC092iC02p01937
    » https://doi.org/10.1029/JC092iC02p01937
  • KELLER, P. S.; CATALÁN, N.; VON SCHILLER, D.; GROSSART, H. P.; KOSCHORRECK, M.; OBRADOR, B. et al Global CO2 emissions from dry inland waters share common drivers across ecosystems. Nature Communications, v. 11, n. 1, 2020. https://doi.org/10.1038/s41467-020-15929-y
    » https://doi.org/10.1038/s41467-020-15929-y
  • KOKIC, J.; SAHLÉE, E.; SOBEK, S.; VACHON, D.; WALLIN, M. High spatial variability of gas transfer velocity in streams revealed by turbulence measurements. Inland Waters, 2018. https://dx.doi.org/10.1080/20442041.2018.1500228
    » https://dx.doi.org/10.1080/20442041.2018.1500228
  • KREMER, J. N.; NIXON, S. W.; BUCKLEY, B.; ROQUES, P. Technical Note: Conditions for Using the Floating Chamber Method to Estimate Air-Water Gas Exchange. Estuaries, v. 26, n. 4A, 2003.
  • LI, S.; MAO, R.; MA, Y.; SARMA, V. V. S. S. Gas transfer velocities of CO2 in subtropical monsoonal climate streams and small rivers. Biogeosciences, v. 16, n. 3, p. 681-693, 2019. https://doi.org/10.5194/bg-16-681-2019
    » https://doi.org/10.5194/bg-16-681-2019
  • LISS, P.; SLATER, P. Flux of Gases across the Air-Sea Interface. Nature, v. 247, p. 181-184, 1974. https://doi.org/10.1038/247181a0
    » https://doi.org/10.1038/247181a0
  • LIU, S.; KUHN, C.; AMATULLI, G.; AHO, K.; BUTMAN, D. E.; ALLEN, G. H. et al The importance of hydrology in routing terrestrial carbon to the atmosphere via global streams and rivers. Earth, Atmospheric, and Planetary Sciences, v. 119, n. 11, 2022. https://doi.org/10.1073/pnas.2106322119
    » https://doi.org/10.1073/pnas.2106322119
  • LOPES DA SILVA, D. M.; OMETTO, J. P. H. B.; LOBO, G. A.; LIMA, W. P.; SCARANELLO, M. A. et al Can Land Use Changes Alter Carbon, Nitrogen and Major Ion Transport In Subtropical Brazilian Streams? Scientia Agricola, v. 64, n. 4, 2007. https://doi.org/10.1590/S0103-90162007000400002
    » https://doi.org/10.1590/S0103-90162007000400002
  • LORKE, A.; BODMER, P.; NOSS, C.; ALSHBOUL, Z.; KOSCHORRECK, M.; SOMLAI-HAASE, C. et al Technical note: drifting versus anchored flux chambers for measuring greenhouse gas emissions from running waters. Biogeosciences, v. 12, p. 7013-7024, 2015. https://doi.org/10.5194/bg-12-7013-2015
    » https://doi.org/10.5194/bg-12-7013-2015
  • MACHADO, D. V.; ALMEIDA, G. S.; MARQUES, E. D.; SILVA-FILHO, E. V. Carbon fluxes in a carbonate rock dominated micro basin of the Quadrilátero Ferrífero, Brazil. Environmental Science and Pollution Research, v. 29, n. 50, p. 76177-76191, 2022. https://doi.org/10.1007/s11356-022-21155-4
    » https://doi.org/10.1007/s11356-022-21155-4
  • MCGILLIS, W. R.; EDSON, J. B.; HARE, J. E.; FAIRALL, C. W. Direct covariance air-sea CO 2 fluxes. Journal of Geophysical Research: Oceans, v. 106, n. C8, p. 16729-16745, 2001. https://doi.org/10.1029/2000JC000506
    » https://doi.org/10.1029/2000JC000506
  • NEU, V.; NEILL, C.; KRUSCHE, A. V. Gaseous and fluvial carbon export from an Amazon forest watershed. Biogeochemistry, v. 105, n. 1, p. 133-147, 2011. https://doi.org/10.1007/s10533-011-9581-3
    » https://doi.org/10.1007/s10533-011-9581-3
  • PERSSON, L.; CARNEY ALMROTH, B. M.; COLLINS, C. D.; CORNELL, S.; DE WIT, C. A.; DIAMOND, M. L. et al. Outside the Safe Operating Space of the Planetary Boundary for Novel Entities. Environmental Science and Technology, v. 56, n. 3, p. 1510-1521, 2022. https://doi.org/10.1021/acs.est.1c04158
    » https://doi.org/10.1021/acs.est.1c04158
  • RASERA, M. F. F. L.; BALLESTER, M. V. R.; KRUSCHE, A. V.; SALIMON, C.; MONTEBELO, L. A. et al Estimating the surface area of small rivers in the southwestern amazon and their role in CO2 outgassing. Earth Interactions, v. 12, n. 6, 2008. https://doi.org/10.1175/2008EI257.1
    » https://doi.org/10.1175/2008EI257.1
  • RASERA, M. DE F. F. L.; KRUSCHE, A. V.; RICHEY, J. E.; BALLESTER, M. V. R.; VICTÓRIA, R. L. Spatial and temporal variability of pCO2 and CO2 efflux in seven Amazonian Rivers. Biogeochemistry, n. 116, n. 1-3, p. 241-259, 2013. https://doi.org/10.1007/s10533-013-9854-0
    » https://doi.org/10.1007/s10533-013-9854-0
  • RAYMOND, P. A.; COLE, J. J. Technical Notes and Comments Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity. Estuaries, v. 24, n. 2, 2001.
  • RAYMOND, P. A.; ZAPPA, C. J.; BUTMAN, D.; BOTT, T. L.; POTTER, J.; MULHOLLAND, P. et al Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers. Limnology and Oceanography: Fluids and Environments, v. 2, n. 1, p. 41-53, 2012. https://doi.org/10.1215/21573689-1597669
    » https://doi.org/10.1215/21573689-1597669
  • RICHEY, J. E.; MELACK, J. M.; AUFDENKAMPE, A. K.; BALLESTER, V. M.; HESS, L. L. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, v. 416, n. 6881, p. 617-620, 2002. https://doi.org/10.1038/416617a
    » https://doi.org/10.1038/416617a
  • ROCKSTRÖM, J.; STEFFEN, W.; NOONE, K.; PERSSON, Å.; CHAPIN, F. S.; LAMBIN, E. F. et al A safe operating space for humanity. Nature, v. 461, n. 7263, p. 472-475, 2009. https://doi.org/10.1038/461472a
    » https://doi.org/10.1038/461472a
  • ROSA, M. B. S. DA; FIGUEIREDO, R. D. O.; MARKEWITZ, D.; KRUSCHE, A. V.; COSTA, F. F.; GERHARD, P. Evasion of CO2 and dissolved carbon in river waters of three small catchments in an area occupied by small family farms in the eastern Amazon. Revista Ambi & Agua, v. 12, n. 4, p. 556, 2017. https://doi.org/10.4136/ambi-agua.2040
    » https://doi.org/10.4136/ambi-agua.2040
  • SALIMON, C.; DOS SANTOS SOUSA, E.; ALIN, S. R.; KRUSCHE, A. V.; BALLESTER, M. V. Seasonal variation in dissolved carbon concentrations and fluxes in the upper Purus River, southwestern Amazon. Biogeochemistry, v. 114, n. 1-3, p. 245-254, 2013. https://doi.org/10.1007/s10533-012-9806-0
    » https://doi.org/10.1007/s10533-012-9806-0
  • SAWAKUCHI, H. O.; NEU, V.; WARD, N. D.; BARROS, M. DE L. C.; VALERIO, A. M.; GAGNE-MAYNARD, W. et al Carbon Dioxide Emissions along the Lower Amazon River. Frontiers in Marine>Science, v. 4, 2017. https://doi.org/10.3389/fmars.2017.00076
    » https://doi.org/10.3389/fmars.2017.00076
  • SCOFIELD, V.; MELACK, J. M.; BARBOSA, P. M.; AMARAL, J. H. F.; FORSBERG, B. R.; FARJALLA, V. F. Carbon dioxide outgassing from Amazonian aquatic ecosystems in the Negro River basin. Biogeochemistry, v. 129, n. 1-2, p. 77-91, 2016. https://doi.org/10.1007/s10533-016-0220-x
    » https://doi.org/10.1007/s10533-016-0220-x
  • SORRIBAS, M. V.; DA MOTTA MARQUES, D.; CASTRO, N. M. DOS R.; FAN, F. M. Fluvial carbon export and CO2 efflux in representative nested headwater catchments of the eastern La Plata River Basin. Hydrological Processes, v. 31, n. 5, p. 995-1006, 2017. https://doi.org/10.1002/hyp.11076
    » https://doi.org/10.1002/hyp.11076
  • SOUSA, E.; SALIMON, C. I.; DE OLIVEIRA FIGUEIREDO, R.; KRUSCHE, A. V. Dissolved carbon in an urban area of a river in the Brazilian Amazon. Biogeochemistry, v. 105, n. 1-3, p. 159-170, 2011. https://doi.org/10.1007/s10533-011-9613-z
    » https://doi.org/10.1007/s10533-011-9613-z
  • STEFFEN, W.; RICHARDSON, K.; ROCKSTRÖM, J.; CORNELL, S. E.; FETZER, I.; BENNETT, E. M. et al Planetary boundaries: Guiding human development on a changing planet. Science, v. 347, n. 6223, 2015. https://doi.org/10.1126/science.1259855
    » https://doi.org/10.1126/science.1259855
  • TANG, W.; XU, Y. J.; NI, M.; LI, S. Land use and hydrological factors control concentrations and diffusive fluxes of riverine dissolved carbon dioxide and methane in low-order streams. Water Research, v. 231, n. 119615, 2023. https://doi.org/10.1016/j.watres.2023.119615
    » https://doi.org/10.1016/j.watres.2023.119615
  • ULSETH, A. J.; HALL, R. O.; BOIX CANADELL, M.; MADINGER, H. L.; NIAYIFAR, A.; BATTIN, T. J. Distinct air-water gas exchange regimes in low- and high-energy streams. Nature Geoscience, v. 12, n. 4, p. 259-263, 2019. https://doi.org/10.1038/s41561-019-0324-8
    » https://doi.org/10.1038/s41561-019-0324-8
  • WARD, N. D.; BIANCHI, T. S.; MEDEIROS, P. M.; SEIDEL, M.; RICHEY, J. E.; KEIL, R. G. et al Where carbon goes when water flows: Carbon cycling across the aquatic continuum. Frontiers in Marine Science, v. 4, 2017. https://doi.org/10.3389/fmars.2017.00007
    » https://doi.org/10.3389/fmars.2017.00007
  • WEN, Z.; SHANG, Y.; LYU, L.; LI, S.; TAO, H.; SONG, K. A review of quantifying pco2 in inland waters with a global perspective: Challenges and prospects of implementing remote sensing technology. Remote Sensing, v. 13, n. 13, 2021. https://doi.org/10.3390/rs13234916
    » https://doi.org/10.3390/rs13234916
  • ZAPPA, C. J.; RAYMOND, P. A.; TERRAY, E. A.; MCGILLIS, W. R. Variation in Surface Turbulence and the Gas Transfer Velocity over a Tidal Cycle in a Macro-tidal Estuary. Estuaries, v. 26, n. 6, 2003. https://doi.org/10.1007/BF02803649
    » https://doi.org/10.1007/BF02803649
  • ZHANG, T.; LI, J.; PU, J.; MARTIN, J. B.; WANG, S.; YUAN, D. Rainfall possibly disturbs the diurnal pattern of CO2 degassing in the Lijiang River, SW China. Journal of Hydrology, v. 590, 2020. https://doi.org/10.1016/j.jhydrol.2020.125540
    » https://doi.org/10.1016/j.jhydrol.2020.125540
  • ZIMMER, M. A.; MCGLYNN, B. L. Lateral, Vertical, and Longitudinal Source Area Connectivity Drive Runoff and Carbon Export Across Watershed Scales. Water Resources Research, v. 54, n. 3, p. 1576-1598, 2018. https://doi.org/10.1002/2017WR021718
    » https://doi.org/10.1002/2017WR021718

Supplementary Material

Table S1.
Studies analyzed and used in the review. x = data present in the article.

Equation 1:

F C O 2 = k K H Δ p C O 2 (1)

Where:

FCO2 = flux water-air (mmol m-2 d-1);

k = CO2 gas transfer rate (m d-1);

KH = CO2 solubility (mmol m-3 µatm-1);

ΔpCO2 = pCO2 water - pCO2 atmosphere (µatm)

The CO2 solubility is calculated by Equation 2, where T = temperature in Kelvin.

K H = E X P ( - 58,0931 + 90,5069 ( 100 / T ) + 22,2940 l n ( T / 100 ) ) (2)

CO2 gas transfer rate (k) and the Schmidt number (Sc) (Equations 3, 4, 5, 6, 7 and 8):

S c = v / D (3)

k = S c - n (4)

k 1 / k 2 = ( S c 1 / S c 2 ) - n (5)

k = k 600 ( S c C O 2 / 600 ) - 0.5 (6)

S c = A + B T + C T 2 + C T 3 (7)

S c C O 2 = 1911.1 118.11 T + 3.4527 T 2 0.04132 T 3 (8)

Where:

Sc = Schmidt number

v = water viscosity

D = molecular coefficient of the gas

n = exponent of Schmidt number. Depending on the water surface, where 0.5 beings are used for the river system.

Publication Dates

  • Publication in this collection
    23 Oct 2023
  • Date of issue
    2023

History

  • Received
    10 Mar 2023
  • Accepted
    17 July 2023
Instituto de Pesquisas Ambientais em Bacias Hidrográficas Instituto de Pesquisas Ambientais em Bacias Hidrográficas (IPABHi), Estrada Mun. Dr. José Luis Cembranelli, 5000, Taubaté, SP, Brasil, CEP 12081-010 - Taubaté - SP - Brazil
E-mail: ambi.agua@gmail.com