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Dissolved CO2 Concentrations in the Amazon Basin

Overview

Scientific literature shows that the Amazon River basin is characterized by supersaturation of dissolved CO2, with concentrations varying widely due to differences in hydrology, river size, season, and surrounding landscape. Most research describes concentrations using partial pressure (pCO2, µatm), but these can be reliably converted to mg/ℓ (ppm) for direct comparison.

Note: CO2 (mg/ℓ) = pCO2 (µatm) × 0.00145

Typical Concentrations

Mainstream Amazon River

  • pCO2 values: Typically range from 1,600 to 6,000 µatm in the mainstem during most seasons ^1.
  • Converted to mg/ℓ:
    • Min: ~2.3 mg/ℓ
    • Max: ~8.8 mg/ℓ

Major Tributaries

  • pCO2 values: Range from 70 to 1,070 µatm in major clearwater tributaries, with some seasonal and regional variation ^1 ^2.
  • Converted to mg/ℓ:
    • Min: ~0.10 mg/ℓ
    • Max: ~1.55 mg/ℓ

Floodplains & Small Streams

  • Floodplain waters and some smaller streams can reach even higher concentrations, particularly in oxygen-depleted (anoxic) zones, sometimes exceeding 200–300 µM (corresponds to ~8.8–13.2 mg/ℓ CO2), and in extreme cases, up to 1000 µM (~44 mg/ℓ CO2) ^3.
  • Lowland tributaries generally have dissolved CO2 from 0.1 to 4 mg/ℓ, with higher values associated with floodplain and blackwater systems, especially during high water seasons ^4 ^2.

Summary Table

Water TypepCO2 (µatm)Dissolved CO2 (mg/ℓ)
Mainstem Amazon River1,600–6,0002.3–8.8
Major Tributaries70–1,0700.10–1.6
Small/anoxic streamsup to 1000 µMup to ~44
*Conversions use standard Henry’s Law constants at 25°C.

Notes

  • Seasonality: Highest CO2 concentrations are typically found during high-water periods in the mainstem and floodplain, with tributary concentrations more variable and sometimes below atmospheric equilibrium during low water ^2.
  • Spatial Variability: Mainstem and blackwater systems have higher CO2 than clearwater tributaries, reflecting greater terrestrial carbon inputs and slower exchange with the atmosphere ^2.
  • Research Context: CO2 values fluctuate locally and seasonally, so these numbers represent typical reported ranges across the Amazon basin in recent peer-reviewed studies.

In summary:

The main Amazon River routinely holds 2–9 mg/ℓ dissolved CO2, while its major tributaries are generally lower, at 0.1–1.6 mg/ℓ. Extreme conditions in floodplains or isolated streams can yield higher concentrations, occasionally exceeding 10 mg/ℓ ^1 ^3 ^2.

Sources

Key References

Other References → click to show …

Examples

Negro River Basin

Scofield, V., Melack, J.M., Barbosa, P.M. et al. Carbon dioxide outgassing from Amazonian aquatic ecosystems in the Negro River basin. Biogeochemistry 129, 77–91 (2016). DOI: 10.1007/s10533-016-0220-x

Dissolved organic carbon (DOC) and pCO2 in each tributary sampled in Negro basin and in four sampling points in the Negro River main stem (Negro 1–4):

Rio Negro (tab)
Rio Negro (map)

Sampling points in the study area: A–D represent points in Negro River (Negro 1–4, respectively). The tributaries are: Marauia (1), Tea (2), Uneiuxi (3), Aiuana (4), Urubaxi (5), Daraá (6), Preto (7), Paoari (8), Arirarrá (9), Aracá (10), Demini (11), Cuiuni (12), Jufari (13), Caurés (14), Branco (15), Jauaperi (16), Unini (17), Jaú (18), Puduari (19), Apuaú (20) and Cuieiras (21).

Negro River Basin (2)

Amaral, J.H.F., Farjalla, V.F., Melack, J.M. et al. Seasonal and spatial variability of CO2 in aquatic environments of the central lowland Amazon basin. Biogeochemistry 143, 133–149 (2019). DOI: 10.1007/s10533-019-00554-9

Map showing sampling sites (n = 46) in the lowland Amazon basin:

Rio Negro (map)

N1–N4 represent points in Negro river mainstem and S1–S7 represent points in Amazon-Solimões river mainstem. The tributaries in the Negro basin are: Marauia (1), Tea (2), Uneiuxi (3), Aiuanã (4), Urubaxi (5), Darahá (6), Preto (7), Padauari (8), Arirahá (9), Aracá (10), Demeni (11), Cuiuni (12), Caurés (13), Jufari (14), Branco (15), Unini (16), Jauperi (17), Jaú (18), Puduari (19), Apuaú (20) and Cuieiras (21). The tributaries in the Amazon- Solimões Basin are: Jutaí (22), Juruá (23), Japurá (24), Purus (25), and Madeira (26), L1-L9 represent lake sampling points in the Amazon- Solimões basin: Paupixuna (L1), Curupira (L2), Tefé (L3), Coari (4), Mamiá (L5), Ananás (L6), Cabaliana (L7), Calado (L8) and Tia Dora (L9).

Rio Negro (tab)

Amazon River Basin

Kosten, S. et al. Mapping variability in CO2 saturation in Amazonian rivers at a large spatial scale indicates undersaturated areas. Acta Limnologica Brasiliensia, 2023, vol. 35, e3. DOI: 10.17026/dans-zmd-stsg (full-text available at scielo.br/j/alb/a/ncHgLyNgYLLLVdcbYNJwJLK/)

The spatial distribution of partial CO2 pressure (pCO2) in Amazonian rivers:

Amazon Basin

Abbreviations of main river names as follows: Am - Amazon, Jn - Juruena, Ju - Juruá, Ma - Madeira, Ne - Negro, Pu - Purus, So - Solimões, Ta - Tapajós, Tr - Trombetas, Xi - Xingu.

The above figure gives us a rough idea of the number of locations where CO2 concentrations exceed the equilibrium state (±0.6 mg/ℓ). More precise values for these concentrations (i.e., how much they exceed the equilibrium state) can be found in the following figure:

Partial CO2 pressure (pCO2) in the different sub-basins during high and low-waters:

Amazon Basin (pCO<sub>2</sub>)

Partial CO2 pressure (pCO2) in the different sub-basins during high and low-waters. The number of locations sampled during high and low- waters, respectively, are given in parentheses. The horizontal line is the pCO2 in water in equilibrium with the atmosphere (~380 µatm = 0.55 mg/ℓ). Boxes depict the 25% intervals around the median (black line), and whiskers represent the 25th and the 75th percentile.

With the help of artificial intelligence (perplexity.ai), I have compiled the following table from the above figure (and accompanying data):

Amazon Basin (CO<sub>2</sub>)

Dissolved CO2 in Amazon Sediment Pore Water

Overview

Direct measurements of dissolved CO2 in soil solution (pore water) beneath the Amazon River and its tributaries are less common in the literature than surface water values. However, several scientific studies—particularly those focusing on hydrology, groundwater, wetland biogeochemistry, and riverine carbon cycling—provide key insights and concentration ranges.

Reported Pore Water (Soil Solution) CO2 Concentrations

Typical Ranges

  • Floodplain and Sediment Pore Waters:
    Studies indicate that dissolved CO2 in pore waters of floodplain and sediment environments in the Amazon basin can reach much higher concentrations than in overlying river water. Reported values for sediment pore water or groundwater can be:
    • 200–300 μM (micromolar)—a common range in aerobic floodplain soils ^1.
    • Up to 1,000 μM (micromolar) in oxygen-depleted/anoxic sediments ^1.
      • Conversions:
        • 200 μM CO2 ≈ 8.8 mg/ℓ
        • 1,000 μM CO2 ≈ 44 mg/ℓ
  • Small Streams and Groundwater Inputs:
    Headwater studies find emergent groundwater and pore water entering small Amazon streams to be supersaturated with terrestrially respired CO2:
    • Concentrations of 30,000–90,000 ppmv have been measured in deep soil pore spaces in the basin ^2.
    • Much of the riverine CO2—especially in small streams and during baseflow—is derived from these highly concentrated pore waters ^3 ^2.

Influence on Plant-Available CO2

  • Benthic (Sediment) Zone:
    Pore water CO2 levels in the upper 10–30 cm of saturated Amazon sediments are consistently an order of magnitude higher than in river water (which typically ranges 2–9 mg/ℓ).
    • Shallow pore water: commonly 10–40 mg/ℓ CO2, sometimes higher under anoxic or recently inundated conditions ^1 ^2.
  • Spatial and Seasonal Variation:
    • Highest values are observed in anoxic floodplain or blackwater sediments.
    • Fluctuations occur with flood pulse, oxygen availability, and microbial respiration rates.

Summary Table: Dissolved CO2 Concentrations

EnvironmentTypical Range (mg/ℓ)Notes
River water (mainstem)2–9Typical Amazon River surface water ^1
Tributary surface water0.1–4Lower in clearwaters, higher in blackwaters
Sediment/pore water10–44Highest in floodplain/anoxic sediments ^1 ^2
Groundwater (deep soils)Up to 90,000 ppmvEquivalent to several hundred mg/ℓ ^2

Key findings:

  • Sediment and pore water in the Amazon basin often contain 10–40 mg/ℓ (and occasionally up to ~44 mg/ℓ) dissolved CO2, providing a substantially higher source for aquatic plant roots than is found in overlying river water ^1 ^2.
  • CO2 levels are highly variable, driven by organic matter decomposition, hydrological connectivity, and sediment oxygen status.
  • Such elevated pore water CO2 supports the high productivity and growth rates of tropical aquatic plants adapted to these systems.

Sources

Key References

These data confirm that sediment pore water in the Amazon basin can be exceptionally rich in dissolved CO2, far surpassing the concentrations measured in the overlying river water, and forming a critical pool for aquatic plant uptake.

Other References → click to show …

Dissolved CO2 in Mainstems and Tributaries of Mekong, Yangtze, Congo, and Other Tropical Rivers

Dissolved carbon dioxide (CO2) concentrations in rivers vary widely due to influences like organic matter decomposition, hydrology, and connection with floodplain/groundwater sources. Most tropical rivers are supersaturated in CO2 compared to atmospheric equilibrium, often exceeding concentrations seen in temperate systems. Below is a summary of available data from key tropical rivers.

Mekong River Basin

  • Surface Water (Mainstem & Tributaries):
    • Typical partial pressures (pCO2) in the Mekong mainstem and tributaries are 1,000–4,000 µatm, occasionally higher during peak organic matter input.
    • Converted to mg/ℓ (25°C): ≈1.5–5.8 mg/ℓ.
  • Seasonal Variation: Higher CO2 levels during the wet season due to increased terrestrial runoff and in-stream respiration ^1 ^2.

Yangtze River (Changjiang) Basin

  • Surface Water:
    • Median pCO2 in the Yangtze’s mainstem and tributaries: ~2,662 ± 1,240 µatm.
    • Converted to mg/ℓ: ≈3.8 ± 1.8 mg/ℓ.
    • Headwater streams and tributaries can reach transient peaks >5,000 µatm (up to ~7–8 mg/ℓ), especially after heavy rainfall or during organic matter pulses ^3 ^4.

Congo & African Rivers

Congo River:

  • Mainstem: Reported pCO2 typically up to 5,000 ppm.
  • Converted to mg/ℓ: ≈7.3 mg/ℓ.
  • Floodplain Lakes & Tributaries: Floodplain and small tributary waters can rise to much higher pCO2, sometimes exceeding 10,000–20,000 ppm (15–30 mg/ℓ), reflecting strong terrestrial/floodplain CO2 inputs ^5 ^6.

Other African Rivers:

  • Data from surveys of multiple sub-Saharan river basins indicate mean surface water pCO2 commonly 1,000–5,000 µatm (≈1.5–7.3 mg/ℓ), with elevated concentrations in wetlands, groundwater inputs, and anoxic tributaries ^7.

Sediment (Pore Water) CO2

  • In all these tropical systems, sediment/pore water CO2 concentrations are substantially higher than in overlying river water, due to active microbial decomposition in anoxic conditions.
    • Typical ranges for pore water CO2 in floodplains and river sediments: 10–44 mg/ℓ, occasionally exceeding 100 mg/ℓ in highly organic, anoxic environments.
    • Groundwater and deep sediment gases may reach up to 90,000 ppmv CO2—equivalent to ~130 mg/ℓ dissolved at equilibrium with river pore water at 25°C ^7.

Comparative Table: Dissolved CO2 in Key Tropical Rivers

River SystemTypical Surface pCO2 (µatm/ppm)CO2 (mg/ℓ, surface)CO2 (mg/ℓ, pore water/sediment)
Amazon (reference)1,600–6,000 µatm2.3–8.810–44 (up to 130 mg/ℓ locally)
Mekong1,000–4,000 µatm1.5–5.810–44
Yangtze2,662 ± 1,240 µatm3.8 ± 1.810–44
Congo (African)up to 5,000 ppmup to 7.310–44 (30–130 mg/ℓ occasionally)
Other African Rivers1,000–5,000 µatm1.5–7.310–44

Key Points:

  • Mainstream and large tributaries of tropical rivers typically hold surface dissolved CO2 of 1.5–8 mg/ℓ, with higher values possible in organic-rich or stagnant water bodies.
  • Sediment pore waters generally contain 10–44 mg/ℓ CO2, but may reach over 100 mg/ℓ in anoxic zones.
  • CO2 levels are seasonally dynamic and strongly influenced by the hydrological regime, organic carbon input, and the connectivity of floodplains and wetlands.

Sources

Key References

These values provide a robust comparative context for dissolved and pore water CO2 concentrations across the world’s tropical river systems.

Other References → click to show …

Dissolved Oxygen in Congo Floodplain Lakes & High-CO2 Tributaries

What concentrations of dissolved oxygen (O2) can be expected in those floodplain lakes & tributaries of Congo River where CO2 concentrations occasionally exceed 15-30 mg/ℓ?

Overview

Floodplain lakes and lowland tributaries of the Congo River that display exceptionally high dissolved CO2 concentrations (15–30 mg/ℓ and above) are characterized by intense heterotrophic metabolism, strong organic matter inputs, and often poor water exchange with the mainstem. These factors profoundly impact dissolved oxygen (DO) levels.

Expected Dissolved Oxygen Ranges

  • Typical DO Concentrations:
    • In areas with CO2 > 15–30 mg/ℓ, DO is frequently very low due to the dominance of microbial respiration and decomposition, particularly during the flood pulse when waters are cut off (or only weakly connected) to the main river channel.
    • Observed ranges:
      • <2.0 mg/ℓ is common, with many sites at or near hypoxic (<2 mg/ℓ) or anoxic (<0.5 mg/ℓ) levels.
      • During peak flooding, DO values can fall to 0.1–0.5 mg/ℓ in backwaters and seasonally isolated areas draining dense flooded forests or wetlands ^1 ^2 ^3 ^4.
  • Oxygen Saturation (%):
    • Measurements of %O2 saturation show that these waters are often <15–30% of air saturation, and sometimes virtually undetectable.

Process Explanation

  • High CO2, Low O2 Link:
    • CO2 accumulation is driven by intense organic matter breakdown (microbial and plant respiration), which rapidly consumes oxygen—as oxygen is depleted, anaerobic processes and CO2 production continue, especially in waters with lots of decomposing plant debris.
    • Major drivers: floodplain connectivity, water stagnation, input of terrestrially derived carbon, and the proportion of aquatic plant and microbial communities that generate or consume gases.
    • Tributaries and lakes connected with dense wetland forests and swamps show the most extreme contrasts.

Supporting Data & Regional Comparison

EnvironmentDissolved CO2 (mg/ℓ)Dissolved O2 (mg/ℓ)Notes
Congolese floodplain lakes15–30+<2.0 (often 0.1–1.0)Hypoxic–anoxic, seasonal extremes
Wetland tributaries (CCC*)20–35+~0.2–1.5Lowest O2 in dry/peak flood period
Mainstem Congo2–94–7Generally normoxic
*CCC = Cuvette Centrale Congolaise, a vast wetland system in the Congo Basin.

Key findings:

  • CO2 concentrations and O2 saturations show strong inverse correlation: the highest CO2 zones have the lowest O2, confirming the expected biogeochemical dynamics of tropical floodplains ^1 ^3 ^4.
  • This pattern is identical to that reported in other tropical flood-pulse systems, such as the Amazon's anoxic várzea and the Tonle Sap floodplains in Southeast Asia ^5 ^6 ^7.

Summary:

Floodplain lakes and tributaries in the Congo River Basin with CO2 concentrations exceeding 15–30 mg/ℓ consistently exhibit very low dissolved oxygen, typically well below 2 mg/ℓ, and are often hypoxic or anoxic for extended periods. This is a direct result of intense biological respiration, organic matter decomposition, and weak exchange with well-oxygenated waters ^1 ^4.

Sources

Key References

These values provide a robust comparative context for dissolved and pore water CO2 concentrations across the world’s tropical river systems.

Other References → click to show …

Submersed Aquatic Macrophytes in Waters with High CO2 and Low O2

Are these waters with high CO2 concentrations and low O2 concentrations rich in sumbersed aquatic macrophytes?

General Patterns

  • High CO2 Concentrations: Elevated dissolved CO2 supports the growth of submersed aquatic macrophytes capable of utilizing CO2 efficiently for photosynthesis. In experimental and field studies, increased CO2 levels often enhance growth, photosynthetic rates, and clonal propagation of submersed species such as Vallisneria natans and other similar plants, provided light conditions are adequate ^1 ^2 ^3.
  • Low O2 Concentrations: Reduced dissolved oxygen (hypoxia or anoxia) is common in tropical floodplain waters with high organic matter decomposition, which often coincides with high CO2 levels. Many wetland and submersed aquatic plants possess physiological adaptations like aerenchyma (gas transport tissues) and efficient anaerobic metabolism, allowing them to tolerate and even thrive under periodic or persistent low O2 conditions ^4 ^5.

Real-World Observations

  • Floodplain Lakes and Tributaries: These habitats, such as those in the Amazon and Congo basins, often have high CO2 and low O2, especially during flood events or in stagnant areas. Submersed macrophytes are present but not always abundant. Their distribution and biomass can be limited by factors like light availability (due to turbidity or water depth), duration of submergence, and competition from emergent or floating plants ^6 ^7 ^3.
  • Dominance Patterns: Open-water areas within floodplains, especially those with less shading and turbidity, are more likely to support greater abundance of submerged macrophytes. In contrast, regions dominated by floating vegetation, high organic content, and persistent anoxia may favor emergent or floating plants over submersed forms ^2.

Environmental Constraints

  • Key Limiting Factors:
    • Light Availability: Decreased light penetration from turbidity, floating macrophytes, or deep water is usually the main constraint on submerged plant dominance, even in CO2-rich waters ^6 ^8 ^2.
    • Prolonged Anoxia: While many submersed species tolerate low O2, extreme or extended anoxia, particularly in combination with high DOC and low light, can limit their abundance and diversity ^8 ^4.
    • Flood Duration: Long flood pulses and water level increases lead to loss of submerged plant biomass and diversity due to poor light and physical disturbance, even if CO2 is abundant ^6 ^8.

Summary Table

ConditionInfluence on Submersed Macrophytes
High CO2, moderate O2, clear waterPromotes high biomass and growth
High CO2, low O2, high turbidity/low lightLimits abundance; a few tolerant species persist
Persistent anoxia (very low O2)Only highly tolerant species may survive
Presence of floating/emergent macrophytes (shading)Reduces submersed plant abundance

Conclusions:

  • Submersed macrophytes can be present in waters with high CO2 and low O2, but their overall abundance depends critically on light availability and the duration/severity of oxygen depletion. The richest communities usually occur in clear, light-penetrating waters, even if CO2 is high. Waters that are both high in CO2 and chronically low in O2 (especially under deep shading or high organic loading) tend to support more limited, stress-tolerant submersed vegetation, or may be dominated by floating and emergent species instead ^6 ^8 ^2 ^4 ^5.

Sources

Key References

Other References → click to show …

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