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FIGURE 12.9 DOC concentrations along a longitudinal transect in the Hudson River, from Albany, New York, at river kilometer 245 toward New York City at kilometer 0. The downstream decrease in DOC concentrations is consistent with a loss of 100 g C m 2 year1 ofallochtho-nous organic C. (Reproduced from Cole and Caraco 2001.)

8-year record, the lower Hudson was almost always supersaturated with CO2 (Cole and Caraco 2001). Analysis of an additional 46 large rivers worldwide found that the majority of large rivers are net sources of CO2 to the atmosphere, evidence of considerable metabolic activity. As one example, the Amazon Basin exports 13 times more C as CO2 to the atmosphere than as DOC or POC to the oceans (Richey et al. 2002). In both the Amazon and the Hudson, the age of organic matter decreases downstream, suggesting that the old C exported from the terrestrial systems is mineralized and replaced by younger C in the lower reaches. Cole and Caraco (2001) estimate that, on average, 43% of the organic C inputs to rivers are converted to CO2 while in transport. Thus, although headwater through mid-order stream ecosystems undoubtedly export large quantities or organic matter to lower river reaches, large rivers may be capable of mineralizing a substantial amount of the C that enters from terrestrial ecosystems.

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