High uptake rates for dissolved organic C are reported for labile molecules including leachate from leaves and highly productive algal mats, and the addition of nutrients often increases DOC uptake (Section 7.3 2). However, the majority of DOC enters streams from soil and groundwater and includes a heterogeneous mix of bioavailable, refractory, and perhaps inhibitory compounds, making total DOC a poor predictor of microbial metabolism. Although the rate of utilization of the DOC pool by hetero-trophs is difficult to estimate, the majority of studies point to biological assimilation as the principal removal process, varying with the fraction of DOC that is labile.
By measuring uptake coefficients of DOC using 13C-labeled leaf leachate in mesocosms, Wiegner et al. (2005) estimated that the most readily assimilated DOC fraction would travel 175 m in White Clay Creek before being immobilized, and a second DOC pool they described as of intermediate lability would travel 3,692 m. These distances represented 7% and 150% of the third-order reach length, respectively, suggesting that readily available DOC was an energy input at the reach scale, whereas less available material was exported and potentially served as a subsidy to downstream ecosystems. Because this experiment used fresh leachate rather than material aged by passage through the soil, uptake distances may be underestimated. However, other studies have found that DOC in transport can support 11-55% of the benthic bacterial metabolism in streams and rivers (Bott et al. 1984, Findlay et al. 1993b, Fischer et al. 2002a, Sobczak and Findlay 2002). In the mesocosms studied by Weigner et al., DOC met from one third to one half of the bacterial C demand, depending on the importance of algal production.
Despite many measures of DOC concentrations and a reasonably good understanding of factors that influence spatial and temporal variation, neither input nor utilization rates are well quantified on an areal basis. At least in small streams, downstream transport rather than utilization appears to be the fate of most DOC entering stream reaches. Over the entire length of rivers, however, owing to greater DOC utilization in large lowland rivers, it appears that a substantial proportion of organic C inputs are indeed converted to CO2. In the lower Hudson River, New York, DOC is the largest pool of organic C, and its concentrations decrease from mile 245, near Albany, toward New York City (Figure 12.9), which implies mineralization owing to heterotrophic activity. Based on an
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