The oxygen‐18 isotope approach for measuring aquatic metabolism in high productivity waters

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We examined the utility of δ18O2 measurements in estimating gross primary production (P), community respiration (R), and net metabolism (P : R) through diel cycles in a productive agricultural stream located in the midwestern U.S.A. Large diel swings in O2 (±200 µmol L−1) were accompanied by large diel variation in δ18O2 (±10‰). Simultaneous gas transfer measurements and laboratory‐derived isotopic fractionation factors for O2 during respiration (αr) were used in conjunction with the diel monitoring of O2 and δ18O2 to calculate P, R, and P :R using three independent isotope‐based methods. These estimates were compared to each other and against the traditional “open‐channel diel O2‐change” technique that lacked δ18O2. A principal advantage of the δ18O2 measurements was quantification of diel variation in R, which increased by up to 30% during the day, and the diel pattern in R was variable and not necessarily predictable from assumed temperature effects on R. The P, R, and P :R estimates calculated using the isotope‐based approaches showed high sensitivity to the assumed system fractionation factor (αr). The optimum modeled ar values (0.986‐0.989) were roughly consistent with the laboratory‐derived values, but larger (i.e., less fractionation) than αr values typically reported for enzyme‐limited respiration in open water environments. Because of large diel variation in O2, P :R could not be estimated by directly applying the typical steady‐state solution to the O2 and 18O‐O2 mass balance equations in the absence of gas transfer data. Instead, our results indicate that a modified steady‐state solution (the daily mean value approach) could be used with time‐averaged O2 and δ18O2 measurements to calculate P :R independent of gas transfer. This approach was applicable under specifically defined, net heterotrophic conditions. The diel cycle of increasing daytime R and decreasing nighttime R was only partially explained by temperature variation, but could be consistent with the diel production/consumption of labile dissolved organic carbon from photosynthesis.