Carbon-13 depletion in a subalpine lake: carbon flow implications.

Plankton (larger than 76 micrometers) taken from an undisturbed oligotrophic was found to have delta(13)C values (=(13)/(12)C relative to the Pee Dee be- belemnite standard) rangingfrom -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.