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 HCO3 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, grampositive 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, clear-water lake. Among lakes, the relative importance of autochthonous versus allochthonous carbon to planktonic consumers is positively correlated to the ratio of color (absorbance of light at 440 nm, an indicator of terrestrially derived organic carbon) to chlorophyll. Aquatic ecosystem carbon budgets are often dominated by large inputs of dissolved and particulate organic carbon from land. This terrestrially derived organic matter partially fuels both system metabolism and secondary production in many streams, rivers, lakes, and estuaries (Webster and Myer 1997; Wetzel 2001; Chanton and Lewis 2002). The relative support of consumers by autochthonous and allochthonous resources, however, is difficult to determine except in rare cases (e.g., Peterson and Howarth 1987). One method for differentiating autochthonous and allochthonous sources is to experimentally manipulate the stable isotope values of one of the pools. We have used this approach in previous work by adding HCO3 to entire lakes to enrich the d13C of dissolved inorganic carbon used by photosynthetic organisms (Cole et al. 2002; Carpenter et 1 Corresponding author ([email protected]).