Long-term spatial and temporal variation of CO2 partial pressure in the Yellow River, China

Other literature type English OPEN
Ran, L. ; Lu, X. X. ; Richey, J. E. ; Sun, H. ; Han, J. ; Yu, R. ; Liao, S. ; Yi, Q. (2015)

Carbon transport in river systems is an important component of the global carbon cycle. Most rivers of the world act as atmospheric CO<sub>2</sub> sources due to high riverine CO<sub>2</sub> partial pressure (<i>p</i>CO<sub>2</sub>). By determining the <i>p</i>CO<sub>2</sub> from alkalinity and pH, we investigated its spatial and temporal variation in the Yellow River watershed using historical water chemistry records (1950s–1984) and recent sampling along the mainstem (2011–2012). Except the headwater region where the <i>p</i>CO<sub>2</sub> was lower than the atmospheric equilibrium (i.e. 380 μatm), river waters in the remaining watershed were supersaturated with CO<sub>2</sub>. The average <i>p</i>CO<sub>2</sub> for the watershed was estimated at 2810 ± 1985 μatm, which is 7-fold the atmospheric equilibrium. As a result of severe soil erosion and dry climate, waters from the Loess Plateau in the middle reaches had higher <i>p</i>CO<sub>2</sub> than that from the upper and lower reaches. From a seasonal perspective, the <i>p</i>CO<sub>2</sub> varied from about 200 μatm to > 30 000 μatm with higher <i>p</i>CO<sub>2</sub> usually occurring in the dry season and lower <i>p</i>CO<sub>2</sub> in the wet season (at 73% of the sampling sites), suggesting the dilution effect of water. While the <i>p</i>CO<sub>2</sub> responded exponentially to total suspended solids (TSS) export when the TSS concentration was less than 100 kg m<sup>−3</sup>, it decreased slightly and remained stable if the TSS concentration exceeded 100 kg m<sup>−3</sup>. This stable <i>p</i>CO<sub>2</sub> is largely due to gully erosion that mobilizes subsoils characterized by low organic carbon for decomposition. In addition, human activities have changed the <i>p</i>CO<sub>2</sub> dynamics. Particularly, flow regulation by dams can diversely affect the temporal changes of <i>p</i>CO<sub>2</sub>, depending on the physiochemical properties of the regulated waters and adopted operation scheme. Given the high <i>p</i>CO<sub>2</sub> in the Yellow River waters, large potential for CO<sub>2</sub> evasion is expected and warrants further investigation.
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