Variability and Controls of pCO₂ and Air-Water CO₂ Fluxes in a Temperate River

Measurements of riverine dissolved inorganic carbon, total alkalinity (A_T), pH, and the partial pressure of carbon dioxide (pCO₂) can provide insights into the biogeochemical function of rivers, including the processes that control biological production, chemical speciation, and air-water CO₂ fluxes. The complexity created by these combined processes dictates that studies of inorganic carbon be made over broad spatial and temporal scales. Time-series data like these are relatively rare, however, because sampling and measurements are labor intensive and, for some variables, good measurement quality is difficult to achieve (e.g., pH). In this study, spectrophotometric pH and A_T were quantified with high precision and accuracy at biweekly to monthly intervals over a four-year period (2018–2021) along 216 km of the Upper Clark Fork River (UCFR) in the northern Rocky Mountains, USA. We use these and other time-series data to provide insights into the processes that control river inorganic carbon, with a focus on pCO₂ and air-water CO₂ fluxes. We found that seasonal snowmelt runoff increased pCO₂ and that expected increase and decrease of pCO₂ due to seasonal heating and cooling were likely offset by an increase and loss of algal biomass, respectively. Overall, the UCFR was a small net source (0.08 ± 0.14 mol m⁻² d⁻¹) of CO₂ to the atmosphere over the four-year study period with highly variable annual averages (0.0–0.10 mol m⁻² d⁻¹). The seasonally correlated, offsetting mechanisms highlight the challenges in predicting pCO₂ and air-water CO₂ fluxes in rivers.