Lake Trasimeno is a closed lake known for its shallow waters and serves a valuable sentinel site for assessing climate-driven impacts. It is also a long-term ecological research (LTER) site and has been the subject of naturalistic and ecological research since the last century. The LTER network plays a pivotal role in tracking and forecasting ecosystem responses to environmental variability and anthropogenic pressures. Characterizing the phytoplankton community structure is essential for evaluating ecosystem status and detecting shifts in environmental conditions.

This study focuses on the analysis of a continuous 50-year monthly time series of phytoplankton data. Our findings reveal significant long-term changes in phytoplankton dynamics, including shifts in species composition, community structure, seasonal succession patterns, and overall diversity. We observed a general shift in dominance toward Cyanobacteria, accompanied by a decline in other algal classes and orders, as well as reduced community evenness.

Salinity and water column stability were identified as the main potential driving forces behind the observed seasonal succession changes. In contrast, temperature and water depth appeared to have an unclear influence, with no consistent or well-defined patterns emerging from the analysis.

Additionally, this study applied the morpho-functional (MFG) groups index as a valuable tool for assessing phytoplankton community structure. Morpho-functional groups classify phytoplankton based on ecological and morphological features. Grouping phytoplankton species by functional traits enhances the understanding of community succession. Furthermore, using indices that consolidate taxonomic information into functional units provides deeper insight into the ecological assessment of the Lake Trasimeno ecosystem.