Although a range of over 30%0 in stable carbon isotope ratios has been recorded for freshwater plankton, no extensive synoptic survey has been undertaken to examine whether any environmental or ecophysiological determinants of 6I3C exist. Here we reanalyze existing literature data on the seasonal variation in plankton F13C from eutrophic lakes, and collect new data on mld-summer zooplankton FI3C from 52 Canadian Shield lakes, to demonstrate that plankton become enriched in 13C relative to ' ' C under conditions of high productivity and low pCO,. Low zooplankton 6I3C signatures in humic/dystrophic lakes of high DOC (dissolved organic carbon) concentration and colour, and in clearwater lakes with high rates of epilimnetic respiration and hypolimnetic metabolism, suggest that the I3C-depletion observed in oligotrophic lakes is probably due to heterotrophic activity.

a n c e t , * , Jeff G. ~o l m q u i s t~ 'WDNRG Limnetics.

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ABSTRACT. Seven colonies of Ivory gulls comprising