| Abstract: | We examined the utility of ??18O2 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 ??mol L-1) were accompanied by large diel variation in ??18O2 (??10???). Simultaneous gas transfer measurements and laboratory-derived isotopic fractionation factors for O2 during respiration (??r) were used in conjunction with the diel monitoring of O2 and ??18O2 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 ??18O2. A principal advantage of the ??18O2 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 (??r). The optimum modeled ??r values (0.986-0.989) were roughly consistent with the laboratory-derived values, but larger (i.e., less fractionation) than ??r 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 ??18O2 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. |
| Genre: | Article |
| ProdID: | 70030850 |
| Citation Author: | Tobias, C. R.; Bohlke, J. K.; Harvey, J. W. |
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| Citation End Page: | 1453 |
| Citation Issue: | 4 |
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| Citation Language: | English |
| Citation Larger Work Title: | Limnology and Oceanography |
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| Citation Number Of Pages: | 15 |
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| Citation Search Results Text: | The oxygen-18 isotope approach for measuring aquatic metabolism in high-productivity waters; 2007; Article; Journal; Limnology and Oceanography; Tobias, C. R.; Bohlke, J. K.; Harvey, J. W. |
| Citation Start Page: | 1439 |
| Citation Volume: | 52 |
| Citation Year: | 2007 |
| Type: | citation/reference |
| Text: | The oxygen-18 isotope approach for measuring aquatic metabolism in high-productivity waters; 2007; Article; Journal; Limnology and Oceanography; Tobias, C. R.; Bohlke, J. K.; Harvey, J. W. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Mon, 1 Jan 2007 00:00 -0600 |
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