Distinct concentration‐discharge dynamics in temperate streams and rivers: <scp>CO<sub>2</sub></scp> exhibits chemostasis while <scp>CH<sub>4</sub></scp> exhibits source limitation due to temperature control
Kelly S. Aho, Jennifer H. Fair, Jacob D. Hosen, Ethan D. Kyzivat, Laura Logozzo, Gerard Rocher‐Ros, L. Weber, Byungman Yoon, Peter A. Raymond
Abstract
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.