Relative contribution of autochthonous and allochthonous carbon to limnetic zooplankton: A new cross-system approach

We measured the C-13 signature of epilimiletic CO2, particulate organic carbon (POC), and zooplankton in 27 lakes in late summer, and used the relationships between the POC and zooplankton signatures and the CO2 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 parts per thousand enriched in C-13 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 C-13. The relationship between C-13-POC and (CO2)-C-13 allowed us to estimate average photosynthetic fraction as -15.9 parts per thousand. This estimate is independent of how much allochthonous C contributes to POC. Given the strength of the relationships between C-13 of POC or zooplankton with (CO2)-C-13 signature, we infer that variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton C-13 signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer.