Coastal marine ecosystems in the Mediterranean are increasingly exposed to multiple pressures, including climate change and local anthropogenic impacts, with consequences for biodiversity and ecosystem functioning. In the Ligurian Sea, previous studies have shown marked changes in water-column temperature dynamics, including longer warm seasons and warmer cold seasons (Vassallo et al, 2022). As zooplankton play a key role in pelagic food webs and biogeochemical cycling, understanding changes in mesozooplankton community structure and functions is essential to assess ecosystem responses to ongoing environmental change.

This study investigates seasonal and long-term dynamics of mesozooplankton communities at the Long-Term Ecological Research (LTER) site Promontory of Portofino (eLTER-IT15-001-M), using a 20-year time series collected between 2003 and 2022. The dataset includes 345 samples collected approximately every 15 days under favourable sea conditions. We applied a food-web modelling approach and Ecological Network Analysis (ENA) to characterise community structure and function, identify key functional nodes, and detect potential trophic reorganisation over time. Mesozooplankton abundance data were converted into biomass estimates using the normalized biomass size spectrum (NBSS), derived from ZooScan imaging and EcoTaxa workflow.

Our results first revealed a marked seasonal pattern in individual size, accompanied by seasonal variation in several ecological network indicators. These observed dynamics highlight higher values of Average Mutual Information and Finn’s Cycling Index in warms seasons while Total System Throughput shown a peak only during spring. We also identified temporal discontinuities in mesozooplankton food-web, particularly around 2009, 2014, and 2018, coinciding with changes in environmental parameters such as temperature, salinity, and nutrient concentrations. These results show how long-term ecological observations, when coupled with food-web modelling, can help interpret plankton dynamics not only as changes in community composition or size structure, but also in terms of ecosystem structure and functions under environmental change.