Mapping variability in CO₂ saturation in Amazonian rivers at a large spatial scale indicates undersaturated areas

Mapeamento da variabilidade na saturação de CO₂ em rios amazônicos em larga escala indica áreas subsaturadas

Abstract

Aim

Amazonian rivers outgas high amounts of carbon dioxide (CO₂). However, the outgassing estimate remains uncertain due to the limited spatial distribution of data on water-atmosphere CO₂ fluxes. So far, the vast extent of the basin and the difficult access to some regions have hampered complete mapping. We hypothesize that 1) CO₂ supersaturation prevails in most Amazonian rivers and 2) undersaturation occurs only at a few locations, particularly during the low-water period.

Methods

To obtain insight on the spatial distribution of partial pressure of CO₂ (pCO₂), an essential determinant of CO₂ fluxes, we analysed water sampled by a specially equipped hydroplane at an unprecedented sizeable spatial scale in the Amazon basin at 419 locations.

Results

Confirming our hypothesis, most rivers were supersaturated with CO₂ concerning the atmosphere. In contrast to our expectations, however, we also found below equilibrium CO₂ concentrations in several tributaries, particularly during low-water periods, when light availability in the water column is highest likely coinciding with the highest aquatic primary production rates. Correlation analyses further indicated that pCO₂ values are positively related to the organic carbon density in the soils, net primary production, and the proportion of flood-able land in the sub-basins.

Conclusions

We conclude that supersaturation with CO₂ occurs in most rivers, but several tributaries are undersaturated. Our findings highlight the different roles that Amazonian rivers can play in the regional carbon cycle. We argue that up-scaling should explicitly incorporate areas with riverine CO₂ uptake, particularly at the sub-basin scale.

Keywords:
pCO₂; spatial variability; autotrophy; hydroplane sampling

Graphical Abstract