Macrophyte changes in transitional water systems: Role of water and sediment parameters, the Venice Lagoon as study case

•Different water and sediment parameters highlighted angiosperm cover and algal biodiversity.•Transparency and salinity drove the dominance of angiosperms and sensitive macroalgae.•High nutrient concentrations, Chl-a and TSS drove the dominance of eutrophic species.•Ulva australis dominated, replacing U. rigida in the less eutrophic areas.•The increase of non-indigenous macroalgae did not affect macrophyte biodiversity. The changes in macrophyte biodiversity and cover from the soft bottoms of 87 stations spread in the entire Venice Lagoon in 2011, 2014, 2018 and 2021 have been analyzed. Results showed a strong macrophyte resilience with an increase in the spread of sensitive macroalgae and aquatic angiosperms, especially Z. noltei and R. cirrhosa, which were not affected by the increase of non-indigenous species (NIS) introduction. The simultaneous analysis of macrophyte variables and the main water and sediment parameters carried out in 2021 highlighted the key role of water transparency and salinity to regulate the vegetation, especially the presence/absence of aquatic angiosperms and sensitive macroalgae. Vice versa, high chlorophyll-a, total suspended solids, nitrogen and silicate concentrations in the water column, and high moisture, low grain-size and phosphorus concentrations in surface sediments favored the presence of opportunistic species, especially Ulva rigida, Gracilariopsis longissima, Agardhiella subulata and Solieria filiformis. The distribution of the aquatic angiosperms and the 41 most widespread macroalgae in association with the main environmental parameters allowed us to highlight their different ecological value, their possible presence/absence and abundance; indeed, their spatial and temporal changes can be excellent tools to determine and predict the ecological status of transitional water systems (TWS). These results carried out in a polyhedric basin such as the Venice Lagoon, composed by a complex of very different microhabitats, can be considered representative of most environmental conditions present in the main TWS of the Italian coastline, and spatial and temporal macrophyte changes can be excellent tools to determine and predict their ecological status evolution.