Does terrestrial organic carbon subsidize the planktonic food web in a clear-water lake?

Allochthonous organic carbon can subsidize consumers in aquatic systems, but this subsidy may only be significant in relatively small systems with high organic matter loading. We tested the importance of allochthonous carbon to consumers in a relatively large (258,000 m2) clear-water lake by adding H13CO3 daily for 56 d. Dissolved inorganic carbon (DIC) was substantially enriched in 13C by the addition, but it was also variable over diel cycles because of exchange with the atmosphere and photosynthesis. By measuring the d13C value of a physically separated phytoplankton concentrate as well as the d13C of phospholipid fatty acids, we were able to follow 13C-labeling dynamics of specific groups of phytoplankton and bacteria. The d13C values of particulate organic carbon (POC), dissolved organic carbon (DOC), phytoplankton, bacteria, zooplankton, and the invertebrate predator, Chaoborus spp. all increased to a maximum during the addition and declined once the addition ceased. Autochthony ( % C derived from internal primary production) of carbon pools (POC, DOC) and consumers was assessed by fitting dynamic models to time series of d13C. Autochthonous carbon was the dominant source (88–100%) for POC, gram-positive bacteria, a copepod, zooplankton biomass, and Chaoborus spp. Autochthonous carbon provided a lower fraction (,70%) of carbon to DOC, gram-negative bacteria, and cladoceran zooplankton. In comparison to smaller and more humic lakes, terrestrially derived allochthonous C was less significant to the pelagic food web in this larger