Carbon and Nitrogen Isotopic Signatures of Zooplankton Taxa in Five Small Subalpine Lakes along a Trophic Gradient

Interest in Stable Isotopes Analyses (SIA) is increasing in freshwater ecology to better clarify ecosystems' functioning. By measuring carbon and nitrogen isotopic signatures, food sources and organism trophic levels can be tracked, providing quantitative estimates of bi-dimensional niches. In order to describe some general patterns of carbon and nitrogen stable isotope signatures in lakes, we applied SIA to zooplankton community in five subalpine lakes sampled in spring and summer along a trophic gradient (from oligotrophy to hypereutrophy). Within zooplankton taxa, temporal variation in food sources and trophic levels were compared to find out taxon-specific patterns. Carbon and nitrogen isotopic signatures differed among the five lakes, reflecting depth, topography, and trophic status of the lakes. Carbon isotopic signatures varied more considerably in deeper and larger lakes (Mergozzo and Pusiano) than in a shallower and smaller lake (Lake Endine). Nitrogen isotopic signatures were generally more enriched in lakes Pusiano and Moro than in Lake Mergozzo, whereas in summer, they were depleted in all lakes. These observations indicate that zooplankton taxa specific trophic roles differed among lakes and in time.