Interpreting Seasonal Patterns and Long-Term Changes of Zooplankton in a Deep Subalpine Lake Using Stable Isotope Analysis

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 delta C-13 signatures, giving an insight into food sources and niche partition. In particular, cyclopoids had a restricted range with more negative delta C-13 parts per thousand values and an increase in delta C-13 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 delta C-13 values of the zooplankton community and of Daphnia were positively related to water temperature (R-2 = 0.58 p < 0.0001 and R-2 = 0.68 p < 0.0001, respectively), and the delta C-13 Daphnia signature was positively related to chlorophyll a (R-2 = 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.

Automated summary

The purpose of this study was to investigate the seasonal and interannual variations in carbon and nitrogen stable isotopes in the zooplankton crustacean community of Lake Maggiore in Italy. Different taxa and groups of zooplankton showed distinct carbon isotopic signatures, providing insights into their food sources and niche partitioning. The study found that water temperature and chlorophyll a were positively correlated with the carbon isotopic signatures of the zooplankton community. Moreover, the changing environmental parameters matched the trends in carbon isotopic signatures of certain zooplankton taxa. These results suggest that climate warming affects the availability of food sources and environmental conditions in Lake Maggiore, thereby influencing the isotopic signatures of zooplankton.