Daily CO2 partial pressure and CO2 outgassing in the upper Yangtze River basin: A case study of the Longchuan River, China

Abstract

Summary Rivers have been under sampled to investigate carbon degassing, especially in the tropical and sub-tropical regions. An unprecedented high-temporal-resolution (daily) sampling during July 2008–August 2009 was conducted in the Longchuan River of the upper Yangtze basin, a subtropical monsoon river in China, to reveal the daily-to-seasonal dynamics of the partial pressure of CO2 (pCO2) and CO2 degassing flux from the river using Henry’s constant and CO2SYS. The pCO2 levels ranged from 230 to 8300 μatm with an average of 1230 μatm and obvious daily and seasonal variations. More than 92% samples were supersaturated with CO2 in contrast to the atmospheric equilibrium (380 μatm). pCO2 values in the river water in the wet season were relatively low, except in the flooding event in November, due to a dilution effect by heavy rainfall. In contrast, the pCO2 levels in the dry season were much higher, due to lower pH resulted from anthropogenic activities. Net CO2 degassing and pCO2 were strongly correlated with dissolved nitrogen, but weakly with water temperature, dissolved inorganic carbon and water discharge, and uncorrelated with particulate nutrients and biogenic elements. The estimated water-to-air CO2 degassing flux in the Longchuan River was about 27 mol/m2/yr, with the upper limit of 50 mol/m2/yr. Our study also indicated that among the carbon remobilized from land to water, around 7% (2800 t C/yr) of the total carbon was emitted to the atmosphere, 42% (17,000 t C/yr) deposited in the river-reservoirs system and 51% (21,000 t C/yr) exported further downstream. High spatial and temporal resolution of estimates of CO2 emission from the world large rivers is required due to extremely heterogeneous catchment characteristics and anthropogenic activities in space and time.