Indicators of Climate-Driven Change in Long-Term Zooplankton Composition: Insights from Lake Maggiore (Italy)

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.