Abstract
A large fraction of the organic carbon derived from land that is transported through inland waters is decomposed along river systems and emitted to the atmosphere as carbon dioxide (CO2). The Amazon River outgasses nearly as much CO2 as the rainforest sequesters on an annual basis, representing ~25% of global CO2 emissions from inland waters. However, current estimates of CO2 outgassing from the Amazon basin are based on a conservative upscaling of measurements made in the central Amazon, meaning both basin and global scale budgets are likely underestimated. The lower Amazon River, from Obidos to the river mouth, represents ~13% of the total drainage basin area, and is not included in current basin-scale estimates. Here, we assessed the concentration and evasion rate of CO2 along the lower Amazon River corridor and its major tributaries, the Tapajos and Xingu Rivers. Evasive CO2 fluxes were directly measured using floating chambers and gas transfer coefficients (k600) were calculated for different hydrological seasons. Temporal variations in pCO2 and CO2 emissions were similar to previous observations throughout the Amazon (e.g. peak concentrations at high water) and CO2 outgassing was lower in the clearwater tributaries compared to the mainstem. However, k600 values were higher than previously reported upstream likely due to the generally windier conditions, turbulence caused by tidal forces, and an amplification of these factors in the wider channels with a longer fetch. We estimate that the lower Amazon River mainstem emits 0.2 Pg C yr-1 within our study boundaries, or as much as 0.48 Pg C yr-1 if the entire spatial extent to the geographical mouth is considered. Including these values with updated basin scale estimates and estimates of CO2 outgassing from small streams we estimate that the Amazon running waters outgasses as much as 1.39 Pg C yr-1, increasing the global emissions from inland waters by 43% for a total of 2.9 Pg C yr-1. These results highlight a large missing gap in basin-scale carbon budgets along the complete continuum of the Amazon River, and likely most other large river systems, that could drastically alter global scale carbon budgets.
Highlights
Rivers are no longer viewed as passive conduits from land to sea but, rather, play an active role in processing organic carbon derived from land and returning it to the atmosphere as carbon dioxide (CO2) (Cole et al, 2007; Battin et al, 2009)
The average pressure of CO2 (pCO2) and FCO2 including all seasons and sites measured in the lower Amazon River and its tributaries was 2914 ± 1768 μatm and 6.31 ± 5.66 μmol m−2 s−1, respectively
The outlier data points observed in the Xingu River (Figure 2) for both pCO2 and FCO2 occurred during the high water season (Table 1) likely because of the delivery of different source of water coming from a floodplain area that discharges just upstream of the sampling station during the high water season
Summary
Rivers are no longer viewed as passive conduits from land to sea but, rather, play an active role in processing organic carbon derived from land and returning it to the atmosphere as carbon dioxide (CO2) (Cole et al, 2007; Battin et al, 2009). The remaining organic and inorganic carbon that is exported to the coastal ocean is further processed, released to the atmosphere, or stored in marine waters and sediments (Medeiros et al, 2015; Ibanhez et al, 2016). Data coverage is more sparse, wetlands, which were not included in estimates by Raymond et al (2013), emit another ∼2.1 Pg C year−1 (Aufdenkampe et al, 2011). These combined estimates, along with storage and export terms, imply that roughly 5.7 Pg C year−1 is transported through inland waters, with nearly 75% of this carbon being returned to the atmosphere (Le Quéré et al, 2015). Tropical regions have been identified as hotspots for aquatic CO2 outgassing, representing ∼75% of global emissions, yet they are under-represented in global datasets, with respect to direct measurements of fluxes and concentrations, which allows quantification of gas transfer velocity values that are used in regional and global models (Regnier et al, 2013; Wehrli, 2013)
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