Water-air Carbon Dioxide Exchange and Nutritional Controls in a Typical Karst River

Abstract

To explore water-air carbon dioxide (CO2) exchange and its nutritional controls in karst rivers, water sampling and analyses were conducted in the Furong River as a representative karst river system. Regional hydrometeorological characteristics, carbonate system parameters, total organic carbon (TOC), nutrients and their stoichiometric ratios, and the riverine pressure of aqueous CO2 (pCO2) and its couplings to trophic status were analyzed. The results showed that gas transfer velocity of CO2 (k) varied between 2.71 and 13.0 m·d-1. Riverine pCO2 varied from 78.5 Pa to 21491.2 Pa and was significantly higher in the tributaries than the main stem. Riverine TOC, total nitrogen (TN), and total phosphorus (TP) concentrations were (302.8±50.1), (128.9±67.9), and (0.65±0.98) μmol·L-1, respectively, demonstrating the trophic status of P-limitation. Riverine pCO2 was tightly linked to P-related parameters, suggesting that in-situ metabolism triggered aquatic CO2 supersaturation. Water-air CO2 flux (F) averaged (534.5±801.4) mmol·(m2·d)-1 in the Furong River, which is higher than most of the world's rivers, and thus indicates high potential CO2 emissions.