Stable isotopes indicate individual level trophic diversity in the freshwater gastropod Lymnaea stagnalis

In lacustrine ecosystems, benthic grazers are generally thought to feed on sediment detritus and microalgae, although there is a paucity of information on food resource use within populations. In this study, we investigated individual level trophic signatures for grazing snails, Lymnaea stagnalis, along with primary consumers and producers in the same habitat, using carbon and nitrogen stable-isotope analyses. In addition, we tested whether ontogeny and parasite presence influence food resource use. The large variation in δ¹³C and δ¹⁵N isotopic signatures indicated that individuals within a population feed on different food sources. Snails appear to have much greater individual variance in trophic behaviour than the other lacustrine species sampled (larval chironomid Chironomus plumosus, amphipods, Gammarus lacustris, zooplankton Ceriodaphnia sp. and Simocephalus vetulus). Moreover, variation among snails was not explained by shell length or the presence of parasitic infections. Habitat heterogeneity and resource availability at the microhabitat level may be the primary factors determining individual food sources.