Emerging patterns of CO₂ : O₂ dynamics in rivers and their link to ecosystem carbon processing

Aquatic metabolism is reflected in the dynamics of dissolved oxygen (O 2 ) and carbon dioxide (CO 2 ) concentrations. Thus, paired measurements of CO 2 and O 2 concentrations can capture the metabolic characteristics of an ecosystem, with promising results in lakes. Yet, for rivers, hydrological, chemical, and biological processes all influence CO 2 and O 2 concentrations, complicating how paired measurements can be used to infer ecosystem processes. Here we combine a data synthesis with a simple mechanistic model of river metabolism, gas exchange, groundwater inputs and carbonate equilibrium to assess how each imprints upon CO 2 : O 2 patterns. Among the physicochemical processes considered, groundwater inputs substantially influenced CO 2 : O 2 relationships. Regardless, analysis of paired CO 2 : O 2 data resolved predictable differences in ecosystem function across rivers with variable productivity and disturbance, as well as along the river continuum. Results indicate that paired CO 2 : O 2 data can aid in assessments of river metabolism, provided that we account for the dynamic physical environment.