Functional-based approaches are increasingly being used to define the functional diversity of aquatic ecosystems. In this study, we proposed the use of δ¹³C and δ¹⁵N stable isotopes as a proxy of zooplankton functional traits in Lake Maggiore, a large, deep subalpine Italian lake. We analyzed the seasonal pattern of δ¹³C and δ¹⁵N 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 δ¹³C and δ¹⁵N 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 ¹⁵N enriched group in the lake. The δ¹⁵N 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 δ¹³C and δ¹⁵N stable isotopes represented an effective tool to define ecological roles of freshwater zooplankton species and to determine functional diversity in a lake.

Freshwater zooplankton are a key component of lake food webs and a responsive indicator of changes occurring in an ecosystem’s structure and functioning. A new challenge under climate change is to disentangle the effects of lake warming from changes in lake trophic conditions, and ultimately to relate them to changes in zooplankton and ecosystem functioning. In this study, we examined the zooplankton community of the large deep subalpine Lake Maggiore (Italy) over a period of four decades, spanning changes in both lake trophic conditions and climate warming. Using monthly data from the upper 50 m of water depth, we analyzed long-term trends and investigated the application of zooplankton biomass-based indices in order to provide a better understanding of the changes in the lake ecosystem over time. Examining annual and seasonal patterns of different zooplankton taxa and groups, we observed over time a decreased contribution of Daphnia sp. during the summer and a concurrent increase in microzooplankton, suggesting a change in phytoplankton control in the lake during the recent period. Our study demonstrates that zooplankton communities integrate environmental changes, and underlines the importance of long-term monitoring and the inclusion of seasonality and the entire size range of zooplankton as key components to allow the interpretation of lake ecosystem functioning in response to trophic and climatic changes.

Zooplankton is crucial for the transfer of matter, energy, and pollutants through aquatic food webs. Primary and secondary consumers contribute to the abundance and standing stock biomass, which both vary seasonally. By means of taxa- and size-specific carbon and nitrogen stable isotope analysis, the path of pollutants through zooplankton is traced and seasonal changes are addressed, in an effort to understand pollutant dynamics in the pelagic food web. We analyzed zooplankton plurennial changes in concentration of polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane and its relatives (DDTs) and in taxa-specific δ¹⁵N signatures in two size fractions, ≥450 µm and ≥850 µm, representative of the major part of zooplankton standing stock biomass and of the fraction to which fish predation is mainly directed, respectively. Our work is aimed at verifying: (1) A link between nitrogen isotopic signatures and pollutant concentrations; (2) the predominance of size versus seasonality for concentration of pollutants; and (3) the contribution of secondary versus primary consumers to carbon and nitrogen isotopic signatures. We found a prevalence of seasonality versus size in pollutant concentrations and isotopic signatures. The taxa-specific δ¹⁵N results correlated to pollutant concentrations, by means of taxa contribution to standing stock biomass and δ¹⁵N isotopic signatures. This is a step forward to understanding the taxa-specific role in pollutant transfer to planktivores and of zooplankton enrichment in PCBs and DDTs.

The purpose of this study was to elucidate the patterns and mechanisms driving seasonal and interannual variations of carbon and nitrogen stable isotopes in the zooplankton crustacean community of Lake Maggiore (Italy), during the period 2009–2020. Different zooplankton taxa and groups showed different ranges of δ¹³C signatures, giving an insight into food sources and niche partition. In particular, cyclopoids had a restricted range with more negative δ¹³C‰ values and an increase in δ¹³C fractionation with the establishment of water thermal vertical stratification, highlighting the importance of vertical distribution as a key factor for taxa coexistence in a vertically heterogenous environment. The δ¹³C values of the zooplankton community and of Daphnia were positively related to water temperature (R² = 0.58 p < 0.0001 and R² = 0.68 p < 0.0001, respectively), and the δ¹³C Daphnia signature was positively related to chlorophyll a (R² = 0.32, p < 0.0001). Decomposition of the time-series data for zooplankton carbon and nitrogen signatures and environmental parameters identified increasing trends in water temperature, chlorophyll a and water conductivity and a decrease in nitrate that matched changes in carbon isotopic signature trends in some zooplankton taxa (Bosmina, Daphnia and Diaptomids). Overall, the observed patterns in zooplankton isotopic signatures were interpreted as integrations of the effects of climate warming in Lake Maggiore, affecting both the availability of food sources and environmental conditions.