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

Abstract

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 αr 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. © 2007, by the American Society of Limnology and Oceanography, Inc.