We measured the 13 C signature of epilimnetic CO 2 , particulate organic carbon (POC), and zooplankton in 27 lakes in late summer, and used the relationships between the POC and zooplankton signatures and the CO 2 signature to estimate the average autochthonous contribution to these fractions of the plankton and the average photosynthetic fractionation. Using this novel approach, average autochthonous contribution to epilimnetic POC was between 62 and 75 %. Epilimnetic zooplankton were between 77 and 91 % autochthonous, indicating that zooplankton bias their feeding towards the autochthonous fraction of POC. On average, zooplankton were 1.2‰ enriched in 13 C relative to POC, but their biased feeding on phytoplankton means that they can be depleted relative to POC in lakes where POC is highly depleted in 13 C. The relationship between 13 C-POC and 13 CO 2 allowed us to estimate average photosynthetic fraction as ―15.9‰. This estimate is independent of how much allochthonous C contributes to POC. Given the strength of the relationships between 13 C of POC or zooplankton with 13 CO 2 signature, we infer that variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton 13 C signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer.
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Mohamed et al. (2009) studied this question.