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The $^18 O:^16 O$ of Dissolved Oxygen in Rivers and Lakes in the Amazon Basin: Determining the Ratio of Respiration to Photosynthesis Rates in Freshwaters

P. D. Quay, D. O. Wilbur, J. E. Richey, A. H. Devol, R. Benner and B. R. Forsberg
Limnology and Oceanography
Vol. 40, No. 4 (Jun., 1995), pp. 718-729
Stable URL: http://www.jstor.org/stable/2838307
Page Count: 12
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The $^18 O:^16 O$ of Dissolved Oxygen in Rivers and Lakes in the Amazon Basin: Determining the Ratio of Respiration to Photosynthesis Rates in Freshwaters
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Abstract

The concentration and $^18 O:^16 O$ ratio of dissolved oxygen were measured for 23 rivers and lakes of the Amazon Basin during 1988, 1990, and 1991. With only two exceptions, the rivers and lakes had dissolved oxygen concentrations that were at 20-90% of atmospheric saturation levels. The $\delta^18 O$ of the dissolved oxygen ranged from 15 to 30%. (vs. SMOW). The $\delta^18 O$ for the lakes were the lowest at $15-23%.. $\delta^18 O <24.2%.$ (the atmospheric equilibrium value) are the result of photosynthetic oxygen input. The $\delta^18 O$ of the rivers, in contrast, ranged from 24 to $30%.. \delta^18 O >24.2%.$ resulted from respiration. Despite this clear difference between the $\delta^18 O$ for rivers and lakes, these water bodies had similar levels of oxygen undersaturation. The $\delta^18 O$ and dissolved oxygen concentrations are used to determine the ratio of community respiration (R) to gross photosynthesis (P) rates. R:P varied between $\sim 1 and 1.5$ for lakes and between 1.5 and 4 for rivers. For all rivers and lakes, the measured $\delta^18 O$ indicated the presence of photosynthetically produced oxygen, with the highest proportion occurring in lakes. The $\delta^18 O$ of dissolved oxygen is a unique tracer of photosynthetic oxygen and provides, through a determination of R: P, a means of quantifying the heterotrophic state of freshwaters.

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