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.

中文翻译：

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河流中的呼吸作用分解了溶解氧的稳定同位素；关于温度和流量影响的全球调查

在水生生态系统中量化生态系统呼吸仍然具有挑战性。大多数研究人员假设夜间和白天的呼吸是相等的。最近的研究表明，在有光合作用和无光合作用期间测量溶解氧同位素可以解释白天和夜间呼吸的变化。这些模型对用于呼吸的氧同位素分馏因子 (α) 值极为敏感，但对 α 的变异性和驱动该变异性的因素几乎一无所知。我们使用现场测量、实验室实验和建模方法量化了 α 如何随温度和流速变化。我们使用来自不同生物群落（温带、热带和亚北极）的 16 条河流中的密封循环室在现场测量了 α，以评估一系列可能的 α 值。α 值变化很大，同一生物群落或生态区的不同地点之间的差异（例如，六个沙漠地点之间为 0.9780 ± 0.005 至 0.9898 ± 0.002）比不同生物群落之间的差异更大。我们的数据显示，温度、流量和生物膜特性都会产生 α 的变化，随着温度的降低和流量的增加，直到在高流速下趋于平稳。发生在扩散边界层中的生物和物理过程产生了 α 的变化。我们的结果强调，环境条件会产生可变的 α 值，需要进行特定地点的 α 测量，以及在现场测量 α 时需要考虑的实际意义。