The Diurnal Variation Pattern and Driving Factors of CO2 at the Water-Air Interface in the Open Flow Section of Karst Underground Rivers

Abstract

Abstract The high-intensity pCO2 in karst underground river water is rapidly released or absorbed by aquatic organisms in open flow sections and is an important factor affecting the karst carbon cycle. This article takes a typical karst underground river basin in southwestern China as the research object. By observing the water carbon and water‒air interface exchange fluxes at the outlet of the underground river (Q1), the river section without submerged plants (H1), and the river section with submerged plants (H2), the diurnal variation pattern and driving factors of CO2 flux at the water‒air interface in the karst area were evaluated, and the transformation process of water carbon and the source sink changes in water carbon in the open-flow section were explored. The results show that CO2 flux and pCO2 in the study area exhibit significant spatial differences, with the outlet of the underground river having the highest values. DO is found to be a key controlling factor affecting changes in CO2 flux. The metabolism of submerged plants has a regulatory role on the change in CO2 flux, forming a carbon sink in the river section with submerged plants. Therefore, this study helps to understand the diurnal variation in carbon exchange fluxes at the water‒air interface in the open flow section of karst underground rivers and emphasizes the variation pattern of CO2 fluxes from source to sink in the open flow section, providing an effective way to promote carbon sequestration and sink enhancement in karst water bodies.