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

Abstract

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 m 2 ) clear‐water lake by adding H 13 CO 3 daily for 56 d. Dissolved inorganic carbon (DIC) was substantially enriched in 13 C by the addition, but it was also variable over diel cycles because of exchange with the atmosphere and photosynthesis. By measuring the δ 13 C value of a physically separated phytoplankton concentrate as well as the δ 13 C of phospholipid fatty acids, we were able to follow 13 C‐labeling dynamics of specific groups of phytoplankton and bacteria. The δ 13 C 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 δ 13 C. 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 (

Bibliographic Information