Productivity and heterotrophy influences on zooplankton delta C-13 in northern temperate lakes

Although a range of over 30 parts per thousand 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 delta(13)C exist. Here we reanalyze existing literature data on the seasonal variation in plankton delta(13)C from eutrophic lakes, and collect new data on mid-summer zooplankton delta(13)C from 52 Canadian Shield lakes, to demonstrate that plankton become enriched in C-13 relative to C-12 under conditions of high productivity and low pCO(2). Low zooplankton delta(13)C signatures in humic/dystrophic lakes of high DOC (dissolved organic carbon) concentration and colour, and in clear-water lakes viith high rates of epilimnetic respiration and hypolimnetic metabolism, suggest that the C-13-depletion observed in oligotrophic lakes is probably due to heterotrophic activity.