Respiration in rivers fractionates stable isotopes of dissolved oxygen; a global investigation on the influences of temperature and flow

Quantifying ecosystem respiration remains challenging in aquatic ecosystems. Most investigators assume that nighttime and daytime respiration are equal. Recent studies suggest measuring dissolved oxygen isotopes during periods with and without photosynthesis can account for variations in daytime and nighttime respiration. These models are extremely sensitive to the oxygen isotopic fractionation factor (alpha) value used for respiration, yet almost nothing is known about the variability of alpha and factors driving that variability. We quantified how alpha varies with temperature and flow velocity using field measurements, laboratory experiments, and a modeling approach. We measured alpha in the field using sealed recirculating chambers in 16 rivers from different biomes (temperate, tropical, and sub-arctic) to assess a range of possible alpha values. The alpha values were widely variable, and variation was higher among sites in the same biome or ecoregion (e.g. 0.9780 +/- 0.005 to 0.9898 +/- 0.002 among six desert sites) than across different biomes. Our data revealed that both temperature, flow, and biofilm characteristics produced variations in alpha, with temperature decreasing and flow increasing it, until leveling off at high flow velocities. Biological and physical processes occurring in the diffusion boundary layer produced variations in alpha. Our results highlight that environmental conditions produce variable alpha values, the need for site-specific alpha measurements, and practical implications for consideration when measuring alpha in the field. More generally we illustrate an array of factors that can influence isotopic fractionation associated with metabolic activity of biologically active layers that could be important in any diffusion-limited environment.