To change or not to change: Macrozoobenthic community variation versus persistence in Tortolì lagoon (Sardinia, Italy)

In the framework of the complexity of transitional ecosystem dynamics, which strongly drive ecological processes and their effects on communities and ecosystem functioning, the seasonal changes in 2021 of macrobenthic community were analysed at four sites of the Tortolì lagoon (Sardinian, Italy), which were characterised in terms of temperature, salinity, organic matter content of sediment and dissolved oxygen. Two macrobenthic community conditions separated by 20 years, between 2003 and 2021, were also compared. Both the composition of the community in terms of species and the main functional traits, related to the relationship with the sediment, feeding mode and development type, were investigated. The objective was to identify which aspects (species composition/functional species traits) of the benthic community are most responsible for changes on an annual and multi-year scale, and which persist two decades later. The benthic assemblage was extraordinarily rich in species (86 species in 2021 and 124 species in total over the two years). Most of species were typical of coastal sheltered waters and properly marine, while opportunistic and brackish/halolimnobic species were scarce; two alien Isopods ( Paracerceis sculpta , Mesanthura sp.) and the new record of Enteropneusta ( Glossobalanus minutus ) were also reported. Differences between seasons and sampling sites resulted significant sources of variation and marine species mainly contributed to discriminate sites closer to the sea, while the others were especially distributed in the confined sites. The comparison between 2003 and 2021 highlighted that the species richness remained high in the two periods, but significant changes in the species composition of the benthic community, explained in terms of a 50 % species turnover rate. This was attributed to different dispersal modes, which proved that new colonisers arrived in the lagoon from the adjacent marine area by active (via larvae) and passive (adult rafting, human-mediated activities) means. The persistence in terms of functional group composition and diversity of the macrobenthic community after twenty years was also evidenced. The considerable diversification of functional groups in the benthic food web, together with the high level of species richness and taxonomic diversity, is assumed to be indicative of an efficient degree of ecosystem functioning of the Tortolì lagoon 20 years later. • Temporal study shows 50% species turnover, maintaining high biodiversity. • Stable benthic community structure despite species composition changes over time. • Functional traits of benthic community remained consistent, indicating ecosystem resilience.