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

Abstract

We examined the utility of d 18 O 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 (6200 mmol L 21 ) were accompanied by large diel variation in d 18 O 2 (610%). Simultaneous gas transfer measurements and laboratory-derived isotopic fractionation factors for O 2 during respiration (a r ) were used in conjunction with the diel monitoring of O 2 and d 18 O 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 d 18 O 2 . A principal advantage of the d 18 O 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 (a r ). The optimum modeled a r values (0.986-0.989) were roughly consistent with the laboratory-derived values, but larger (i.e., less fractionation) than a r 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 O 2 and 18 O-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 d 18 O 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.Gross primary production (P) and community respiration (R) are central regulators of biogeochemical turnover in aquatic environments on local to regional scales (del Giorgio et al. 1997;Duarte and Agusti 1998;Cole et al. 2000). The balance between P and R (i.e., net metabolism) provides an indicator of nutrient enrichment, trophic status, allochthonous organic carbon utilization, and overall water quality (del Giorgio and Peters 1994;Hanson et al. 2003). Accurate estimates of P and R are needed to assess carbon, oxygen, and nutrient processing on watershed scales (Mulholland et al. 1997;Sabater et al. 2000;Hall and Tank 2003).In situ estimates of P and R traditionally have relied upon O 2 budgets that balanced sources and sinks of O 2 (e.g., photosynthesis and respiration) with O 2 exchange (invasion or evasion) between water and the atmosphere. For experimental purposes in the current study, as in most previous ...

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