Carbon stable isotopes reveal complex trophic interactions in lake plankton

The lower trophic linkages in lake plankton food webs are generally describ ed as relatively simple, even accounting for the additional complexity of p otential 'microbial looping'. Crustacean zooplankton are frequently amalgam ated into one trophic functional group as grazers of autotrophic production . The carbon stable isotope ratios for separated zooplankton species, parti culate organic matter (POM) and phytoplankton from a number of lakes in Fin land and the UK were analysed. These revealed greater complexity in trophic interactions than would otherwise be observed if the zooplankton had been represented by a mixed sample. Grazing zooplankton were usually depleted in C-13 relative to the bulk POM on which they might feed, with delta(13)C de viating by up to 17 parts per thousand. There were no consistent difference s between delta(13)C values for copepods and cladocerans. Predatory cladoce rans were generally enriched by greater than 1 parts per thousand compared to their putative prey. We suggest that care in separating the zooplankton species for stable isotope analysis may expose otherwise undetected sources of carbon and facilitate unravelling trophic links further up the food web . Copyright (C) 1999 John Wiley & Sons, Ltd.