Spatial variation characteristics and influencing factors of CO₂ partial pressure in the middle reaches of the Yellow River during summer

Abstract

The partial pressure of CO2 in rivers regulates the intensity and direction of carbon dioxide (CO2) exchange at the water–air interface. Environmental, nutrient factors, and their stoichiometric ratios within the watershed may affect the level of carbon dioxide partial pressure in rivers. However, the relationship between pCO2 responses to the environment, nutrient factors, and their stoichiometric ratios at a large watershed scale was not well understood. In this study, water samples from 20 locations in the mainstream and 16 tributaries of the middle reaches of the Yellow River were collected to study the spatial characteristics of pCO2 and its influencing factors. pCO2 vales showed spatial heterogeneity ranging from 45.57 to 3631.48 μatm, and an average of 1341.87 μatm. Relative to the atmospheric equilibrium value (415 μatm), 90% of the samples were supersaturated. During the study period, higher pCO2 appeared in the Wanjiazhai Reservoir at the northernmost part of the mainstream in the middle reaches, and in the tributaries of the Fen, Wei, and Beiluo Rivers. Overall, the transition from carbon sink to carbon source was experienced in the middle reaches of the Yellow River from north to south. River pCO2 exhibited a significant correlation with phosphorus concentrations and their associated stoichiometric ratio, suggesting that microbial metabolism drove CO2 saturation in rivers. In addition, different land use proportions contributed significantly to changes in river pCO2.