Physical and chemical control on CO2 gas transfer velocities from a low-gradient subtropical stream

Abstract

CO2 exchanges across the water–air interface in rivers and lakes are currently believed to be responsible for the dominant share of global aquatic CO2 emissions. The gas transfer velocity (k600) is the key factor that constrains the CO2 fluxes. It is also the most problematic to establish because of its high spatial and temporal variability. Here, we have evaluated the seasonal and spatial dynamics in k600 values and their physical and chemical controlling processes by gas tracer and floating chamber (FC) methods in three reaches of a low-gradient stream channel (Guancun surface stream, ‘GSS’) in a karst terrain in subtropical southwestern China in December 2016 and March, July and September 2017. The k600 values were highly variable in space and time in this small stream. Physical processes, including the velocity of the stream and its slope, were found to control the variations of k600. The k600 values recorded in the dry season (March and December) were at minimal levels due to very slow flow and gentle slope, and were also affected by complexation in the solute-enriched waters. The characteristics high pH and low turbulence of gentle streams in carbonate karst areas are conducive to such complexation, which is of great significance in the limiting CO2 degassing in such regions. We have obtained the first k600 prediction model for small streams in subtropical karst regions. In conclusion, we present a comprehensive approach for predicting the k600 values in small channels by comparison of independent SF6 gas tracer and floating chamber methods.