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Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs


Ecosystems are supported by organic carbon from two distinct sources. Endogenous carbon is produced by photosynthesis within an ecosystem by autotrophic organisms. Exogenous carbon is produced elsewhere and transported into ecosystems. Consumers may use exogenous carbon with consequent influences on population dynamics, predator–prey relationships and ecosystem processes1. For example, exogenous inputs provide resources that may enhance consumer abundance beyond levels supported by within-system primary production2. Exogenous fluxes of organic carbon to ecosystems are often large, but this material is recalcitrant and difficult to assimilate, in contrast to endogenously produced organic matter, which is used more easily3,4. Here we show, by the experimental manipulation of dissolved inorganic 13C in two lakes, that internal primary production is insufficient to support the food webs of these ecosystems. Additions of NaH13CO3 enriched the 13C content of dissolved inorganic carbon, particulate organic carbon, zooplankton and fish. Dynamics of 13C indicate that 40–55% of particulate organic carbon and 22–50% of zooplankton carbon are derived from terrestrial sources, showing that there is significant subsidy of these ecosystems by organic carbon produced outside their boundaries.

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Figure 1: Dynamics of 13C before, during, and after the addition of NaH13CO3 to Paul Lake and Peter Lake.
Figure 2: Comparison between measured and modelled δ 13CPOC in Paul Lake.
Figure 3


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We thank D. Fischer, J. Hinke, C. M. Gille, C. L. Fankhauser, B. Charipar and M. Bastviken for assistance; and N. Haubenstock and R. Drimmie for help and advice on stable isotope analysis. The University of Notre Dame Environmental Research Center provided facilities and support for the field research. This research was supported by a collaborative grant from the National Science Foundation.

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Correspondence to Michael L. Pace.

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Pace, M., Cole, J., Carpenter, S. et al. Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs. Nature 427, 240–243 (2004).

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