Carbon Stable Isotopes Reveal Complex Trophic Interactions in Lake Plankton Jonathan Grey* and Roger I. Jones Department of Biological Sciences, IENS, Lancaster University, Lancaster LA1 4YQ, UK The lower trophic linkages in lake plankton food webs are generally described as relatively simple, even accounting for the additional complexity of potential ‘microbial looping’. Crustacean zooplankton are frequently amalgamated into one trophic functional group as grazers of autotrophic production. The carbon stable isotope ratios for separated zooplankton species, particulate organic matter (POM) and phytoplankton from a number of lakes in Finland 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 13 C relative to the bulk POM on which they might feed, with  13 C deviating by up to 17% There were no consistent differences between  13 C values for copepods and cladocerans. Predatory cladocerans were generally enriched by greater than 1% 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 # 1999 John Wiley & Sons, Ltd. Received 26 January 1999; Revised 18 February 1999; Accepted 18 February 1999 The complexity of trophic links at the lower end of the pelagic food web is still poorly understood. It is well recognised that the traditional concept of a zooplankton community solely reliant on autochthonous production, although applicable in some lake systems, is certainly not so in a large proportion. Subsidies of organic matter from the catchment frequently play an important role at the base of the food web, but are difficult to quantify directly and have thus been previously inferred. We have been using carbon stable isotopes to assess the relative importance of contributions of autochthonous and allochthonous produc- tion to lake food webs and in particular to the crustacean zooplankton. Zooplankton represent an important link from these basal resources to the fish which may have conserva- tion status or commercial worth. This has involved an extensive survey of UK and Finnish lakes of differing allochthonous loading and trophic state. 1,2 . A prerequisite of applying the stable isotope technique to food webs is that the source end-points are sufficiently distinct and robust so as to allow discrimination and tracing of the isotope ratios. In a pelagic food web, phytoplankton and POM (to which phytoplankton contributes to varying degrees) are often assumed to represent the dietary range of the zooplankton present. Since animals typically exhibit  13 C signatures approximately 1% enriched relative to their diet, 3 zooplankton  13 C values may be expected to fall between the values for POM and phytoplankton or within 1% of the least negative (hypothesis 1). A second hypothesis might be that cladocerans would exhibit a carbon signature that more closely reflects that of the POM source as non-selective filter feeders, compared to the more selective, raptorial copepods. Thirdly, the predatory com- ponent of the zooplankton community may exhibit further enrichment of carbon via fractionation up the food chain, and thus complicate interpretation of the trophic links. Here we present data on individual zooplankton species 13 C relative to their putative food sources, highlighting the complexity of links that may be missed by amalgamating the zooplankton into one trophic functional group. METHODS A total of 36 lakes were sampled during 1997–98, ranging in trophic state from small, hypertrophic meres and broads, to large oligotrophic Scottish lochs and incorporating a suite of highly coloured lakes in Finland. Further details regarding lake chemistry can be found in referred papers. 1,2 Samples were collected from near the deepest part of each lake where bathymetry was known. Vertical profiles for temperature and oxygen concentration were determined using a Yellow Springs Instruments probe. (Yellow Springs Instrument Co. Inc., Yellow Springs, DH, USA) to gauge the extent of the mixed layer. Integrated water samples for particulate organic matter (POM, >0.2 mm) were collected from epilimnetic waters using a Friedinger sampler. (Duncan and Associates, Grange-over-Sands, Cumbria, UK) POM was concentrated using a Minitan (Millipore Co., Bedford MA, USA) tangential flow ultrafiltration apparatus fitted with multiple 0.2 mm Durapore (polyvinylidene fluoride) filter plates (Millipore Co.). The concentrated POM was then collected on precombusted Anodisc inorganic filters and dried at 60 °C. Plankton from each lake was collected by vertical hauls through the epilimnion to the surface with a plankton net of mesh 110 mm and stored without preserva- tion. One vertical haul was preserved with 70% industrial methylated spirit (IMS) for zooplankton identification and determination of relative abundance. Crustacean zooplank- ton were picked from the fresh samples by hand, with different species being separated when numbers permitted. Separated zooplankton were maintained in glass fibre/grade *Correspondence to: J. Grey, Department of Biological Sciences, IENS, Lancaster University, Lancaster LA1 4YQ, UK. E-mail: J. Grey@lancaster.ac.uk CCC 0951–4198/99/131311–04 $17.50 Copyright # 1999 John Wiley & Sons, Ltd. RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 13, 1311–1314 (1999)