CO_2 partial pressure and CO_2 emission in the Lower Mekong River

摘要

CO_2 evasion from freshwaters is recently included in assessing global carbon budgets, while large uncertainty of global estimates results from incomplete spatial coverage of aquatic carbon emission. Here we examined the dissolved inorganic carbon (DIC) and carbon dioxide partial pressure (pCO_2) in riverine water from historical records at 46 stations for the period 1972-1998 in the Lower Mekong River (LMR). The river system presented an exceptional temporal and spatial variability of dissolved inorganic carbon (DIC) species. The alkalinity concentration of the LMR varied from 687 to 3189μeq/l with an average of 1524μeq/l (median value: 1586μeq/l), 1.2 times the global median value. Spatial patterns showed diminishing alkalinity downstream at the main-channel stations along the LMR. Similar to many other Himalayan rivers, alkalinity varied seasonally and inversely related to river runoff with a variation factor of 1.2-1.6, despite a water dilution of 7-fold to 13-fold in the summer flood season. The pCO_2 levels ranged from 224 to 5970μatm with a total average of 1090μatm and obvious monthly and spatial variations. Under-saturated pCO_2 samples (<390μatm) primarily occurred in the high-flow period. pCO_2 at the main-channel stations increased downstream and averaged from 720μatm (Mukdahan) to 1670μatm (Can Tho). Calculated water-to-air CO_2 degassing from the Mekong River was 71mol/m~2/yr, thus 6.8Tg C/yr as CO_2 was released to the atmosphere, 1.5-fold the sea-ward DIC export (4.5Tg C/yr). Basin characteristics influences on pCO_2 from 25 global rivers demonstrated that non-carbonate rocks significantly contributed to pCO_2, population density, agricultural practices, elevation and slope decreased pCO_2, while forest significantly increased pCO_2. Our results demonstrated the essential need to consider river water area as an essential atmospheric CO_2 source, albeit this atmospheric flux, representing significant riverine carbon budgets, was highly variable at temporal and spatial scales.