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 (α) value used for respiration, yet almost nothing is known about the variability of α and factors driving that variability. We quantified how α varies with temperature and flow velocity using field measurements, laboratory experiments, and a modeling approach. We measured α in the field using sealed recirculating chambers in 16 rivers from different biomes (temperate, tropical, and sub-arctic) to assess a range of possible α values. The α 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 α, 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 α. Our results highlight that environmental conditions produce variable α values, the need for site-specific α measurements, and practical implications for consideration when measuring α 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.
KeywordsAlpha δ18OO2 Recirculating chambers Diffusion Temperature Water velocity
We thank Bonnie Trejo, Khaliun Sanchir, Loren Secor, Liana Prudencio, Christie Carey, Thomas Harmon, Curtis Gray, Brian Reid, and Lizzy Sisson for their help in the field.
Flavia Tromboni, Walter K. Dodds, Sudeep Chandra and Anne Schechner were funded by NSF Macrosystems, Award 1442562.
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