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

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.