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

We examined the utility of delta O-18(2) 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 O-2 (+/- 200 mu mol L-1) were accompanied by large diel variation in delta O-18(2) (+/- 10%(0)). Simultaneous gas transfer measurements and laboratory- derived isotopic fractionation factors for O-2 during respiration (alpha(r)) were used in conjunction with the diel monitoring of O2 and delta O-18(2) 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 O-2-change'' technique that lacked delta O-18(2). A principal advantage of the delta O-18(2) 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 (alpha r). The optimum modeled ar values (0.986 - 0.989) were roughly consistent with the laboratory-derived values, but larger (i. e., less fractionation) than ar values typically reported for enzyme- limited respiration in open water environments. Because of large diel variation in O-2, P : R could not be estimated by directly applying the typical steady- state solution to the O2 and O-18-O-2 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 O-2 and delta O-18(2) 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.