Distinct concentration‐discharge dynamics in temperate streams and rivers: CO2 exhibits chemostasis while CH4 exhibits source limitation due to temperature control

Abstract Streams and rivers are significant sources of carbon dioxide (CO 2 ) and methane (CH 4 ) to the atmosphere. However, the magnitudes of these fluxes are uncertain, in part, because dissolved greenhouse gases (GHGs) can exhibit high spatiotemporal variability. Concentration‐discharge ( C ‐ Q ) relationships are commonly used to describe temporal variability stemming from hydrologic controls on solute production and transport. This study assesses how the partial pressures of two GHGs— p CO 2 and p CH 4 —vary across hydrologic conditions over 4 yr in eight nested streams and rivers, at both annual and seasonal timescales. Overall, the range of p CO 2 was constrained, ranging from undersaturated to nine times oversaturated, while p CH 4 was highly variable, ranging from 3 to 500 times oversaturated. We show that p CO 2 exhibited chemostatic behavior (i.e., no change with Q ), in part, due to carbonate buffering and seasonally specific storm responses. In contrast, we show that p CH 4 generally exhibited source limitation (i.e., a negative relationship with Q ), which we attribute to temperature‐mediated production. However, p CH 4 exhibited chemostasis in a wetland‐draining stream, likely due to hydrologic connection to the CH 4 ‐rich wetland. These findings have implications for CO 2 and CH 4 fluxes, which are controlled by concentrations and gas transfer velocities. At high Q , enhanced gas transfer velocity acts on a relatively constant CO 2 stock but on a diminishing CH 4 stock. In other words, CO 2 fluxes increase with Q , while CH 4 fluxes are modulated by the divergent Q dynamics of gas transfer velocity and concentration.