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

Long-term datasets are essential tools for tracking and understanding temporal changes in biodiversity at community level.Freshwater zooplankton organisms are a key component of lake food webs and sensitive indicators of changes in ecosystem structure and functioning. A major challenge under climate change is to disentangle the effects of lake warming from changes in lake trophic conditions, and ultimately to relate them to variations in zooplankton communities and overall, to ecosystem functioning. Lake Maggiore (Italy) represents an ideal environment for discerning trophic from climatic impacts, as two distinct phases can be identified: an oligotrophication phase from the 1980s to the 1990s and a more recent phase characterized by water warming with a prolonged thermal stratification.We analyzed long-term data (1981–2019) of zooplankton biomass from Lake Maggiore, a LTER-Italy site (Italian Long-Term Ecosystem Research network), collected through the monitoring program funded by the Commissione Internazionale per la Protezione delle Acque Italo-Svizzere (CIPAIS). Data were collected monthly at the deepest part of the lake, representative of the pelagic zone. Our aim was to evaluate the application of biomass-based zooplankton indices to improve our understanding of temporal changes in the lake ecosystem.Data analyses across different taxonomic groups, at both annual and seasonal scales, revealed a declining summer contribution of Daphnia sp. in recent years and an increase in microzooplankton (nauplii and monogonont rotifers), exhibiting rapid growth and declining phases and possibly failing to control surges in phytoplankton.This study highlights the importance of long-term monitoring, seasonality analysis and the inclusion of the entire zooplankton size spectrum as key components for interpreting lake ecosystem functioning in response to lake trophic and climatic changes. Moreover, our findings suggested that the developments of zooplankton indicators should consider seasonal rather than annual values and that zooplankton biomass represents the most informative parameter, incorporating both abundance and size structure of the zooplankton community.