Abstract
Plankton (larger than 76 micrometers) taken from an undisturbed oligotrophic was found to have δ 13 C values (= 13 C/ 12 C relative to the Pee Dee belemnite standard) ranging from –44 to –47 per mil. This extraordinary 13 C depletion, together with characteristics of the inorganic carbon pool, indicates that lake respiration as well as surrounding soil respiration provide important carbon sources for plankton production in this lake.