This exercise required the class (including the Prof!) to take dissolved oxygen (D.O.) readings at three-hour intervals on Lake Red Rock. We docked at the South Overlook boat Ramp and motored to a point near the mouth of that bay, halfway between shorelines. The boat was outfitted with running lights, and boaters wore PFDs (life jackets) while in the water. The readings are taken for 24 hours straight.
In most water bodies, D.O. is highest in late afternoon, elevated by algal photsynthesis. D.O. falls during the night to a low point near dawn, as the entire biotic community uses oxygen for metabolism. The difference between highs and lows represents net productivity of the system (NPP), and the D.O. level oscillations is the biological community "breathing."
Our Community Metabolism estimates varied considerably, in part because we did not take a late afternoon sample the previous day, so the technique in our lab handbook (Lind) could not be used directly. However, extrapolation of the curve might produce a difference between daytime max and nighttime min of about 9.5-8.5 = 1.0 mg/L NPP. GPP would be in the range of 1.6 mg/L, so respiration is 0.6 mg/L for the day. This would mean a P:R ratio of 2.7 which is extremely high, even for a eutrophic system like Lake Red Rock. This estimate should not be considered reliable. Conducting the study during the summer would be most instructive, as would a comparison with the light-dark bottle method.
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This page prepared by Paul Weihe. Last modified Nov. 2001.