Diel behavior of stable isotopes of dissolved oxygen and dissolved inorganic carbon in rivers over a range of trophic conditions, and in a mesocosm experiment

Rates of diel (24-h) biogeochemical processes in rivers and their effect on daily changes in the concentration of metals and metalloids have been well documented in the literature over the last 20 years. Investigations into the effects of these processes on aquatic systems and the underlying mechanisms that control the processes can significantly improve our understanding of how natural aquatic environments function and will respond to changing environmental conditions and anthropogenic impacts. Daily changes in the rates of biogeochemical processes have, more recently, been shown to influence the stable isotope composition of dissolved oxygen and dissolved inorganic carbon in natural waters. Here we present a comprehensive picture of the persistence and reproducibility of diel cycles of the 18O composition of dissolved molecular oxygen (δ18O-DO) and the 13C composition of dissolved inorganic carbon (δ13C-DIC) across five Montana, USA rivers investigated over a 4-year period. A mesocosm experiment showed the same behavior in δ18O-DO and δ13C-DIC as seen in riverine settings across light and dark periods.A cross plot of δ18O-DO and δ13C-DIC from each stream exhibits a clockwise elliptical pattern which is attributed to the daily changes in the balance of metabolic rates as well as air–water gas exchange. The amplitude of the change in the isotope composition is shown to be directly related to the trophic state of the river and a relationship between net productivity and diel changes in δ18O-DO and δ13C-DIC is presented. This relationship between trophic status with δ18O-DO, δ13C-DIC and production emphasizes the significance of how rates of biogeochemical processes in natural systems can influence the daily changes in the composition of surface waters.