C and N isotopic signatures of zooplankton taxa in five small subalpine lakes along a trophic gradient

Concern on stable isotopes analyses is increasing in freshwater ecology to better clarify ecosystems' functioning. By measuring carbon and nitrogen isotopic signatures, organism food sources and position level along food webs can be tracked, providing quantitative estimates of bi-dimensional niches. In order to verify the forcefulness of some general patterns of carbon and nitrogen stable isotope signatures in lakes of reduced size, we applied stable isotopes analysis to zooplankton community sampled in spring and summer in five small lakes located in the same subalpine region, within a trophic gradient (from oligotrophy to ipereutrophy). Seasonal changes in trophic position and food sources were analysed for the different zooplankton taxa to compare how and whether isotopic signatures and their seasonality are taxa specific. Carbon and nitrogen isotopic signatures were lake- and season- specific, depending on the depth/morphology and the trophic status of each lake, showing already-observed general trends, with some exceptions. Major variations in carbon signature were recorded in Lake Mergozzo and Lake Pusiano, the deepest lakes and with a larger volume, while minor variations appeared in Lake Endine, which has a reduced volume and depth. Carbon was generally less depleted in summer, with the exception of the shallowest Lake Comabbio and, in a lesser extent, of Lake Endine. Nitrogen enrichment was markedly higher in lakes Pusiano and Moro than in Mergozzo, decreasing in all cases in summer time, indicating a great variability of the trophic roles of zooplankton organisms.