Fig. 4: Strong elevation gradients of pCO₂ and CH₄ in streams and rivers across the whole Amazon basin.

Variations of the partial pressure of CO₂ (pCO₂), CH₄ dissolved concentration, and the ratio of CH₄ and CO₂ (CH₄:CO₂) concentration as function of elevation (a, c, e) and catchment slope (b, d, f). Data in the lowland rivers and streams was compiled from literature for pCO₂^{25,35,36,37,39} and CH₄^15,16,31,39. For consistency, only studies based on direct measurements of pCO₂ were selected, excluding data derived from the computation of pCO₂ from pH and total alkalinity, as this computation can provide erroneous and unrealistically high pCO₂ values¹¹³. Data from Scofield et al.³⁸ was excluded because the pCO₂ values reported by these authors in the Negro River (upstream of Manaus) were distinctly lower (710 ppm during low water period and 2608 ppm during higher water period) than other studies in the same system^25,27,29,36 (~2000–3000 ppm during low water period and ~5000-8000 ppm during higher water period). Small symbols show individual measurements, large symbols indicate the median; the blue triangles show the data in the mountainous headwater streams, green triangles show the data in piedmont streams, and the grey circles show the data in the lowland streams and rivers, and black lines indicate linear or polynomial fits to the data. Solid lines show fits to the data (Supplementary Table 4).