Defining Seasonal Functional Traits of a Freshwater Zooplankton Community Using δ13C and δ15N Stable Isotope Analysis

Functional-based approaches are increasingly being used to define the functional diversity of aquatic ecosystems. In this study, we proposed the use of delta C-13 and delta N-15 stable isotopes as a proxy of zooplankton functional traits in Lake Maggiore, a large, deep subalpine Italian lake. We analyzed the seasonal pattern of delta C-13 and delta N-15 signatures of different crustacean zooplankton taxa to determine food sources, preferred habitats, and trophic positions of species throughout one year. The cladocerans Daphnia longispina galeata gr., Diaphanosoma brachyurum, and Eubosmina longispina were grouped into a primary consumer functional group from their delta C-13 and delta N-15 isotopic signatures, but while the former two species shared the same food sources, the latter exhibited a more selective feeding strategy. Cyclopoid copepods occupied a distinct functional group from the other secondary consumers, being the most N-15 enriched group in the lake. The delta N-15 signature of calanoid copepods showed trophic enrichment in comparison to Daphnia and Eubosmina and linear mixing model results confirmed a predator-prey relationship. In our study, we have demonstrated that the use of delta C-13 and delta N-15 stable isotopes represented an effective tool to define ecological roles of freshwater zooplankton species and to determine functional diversity in a lake.