Impact of environmental parameters on macrophyte changes in TWS soft bottoms: the Venice Lagoon as study case

The lagoon of Venice is a large polyhedric basin composed by different microhabitats that represent the main environmental conditions of the Italian transitional water systems (TWS) (Sfriso et al., 2017). Macrophytes and the environmental parameters of the water column and surface sediments, sampled in late spring-early summer 2011, 2014, 2018 and 2021 in 87 stations spread in the entire lagoon, have been analyzed. Globally, 187 macroalgal taxa were found, however the number yearly recorded ranged from 126 to 132 taxa with an annual species variation depending on weather conditions. On average, 32 taxa were found in 20-67% of the stations, whereas 55 in 0-1%. Among them, the number of non-indigenous species (NIS) increased from 2011 (11 taxa) to 2021 (18 taxa) for a total of 21 taxa, without affecting the biodiversity. At the same time the mean coverage of aquatic angiosperms increased significantly, ranging from 19.3% (56 Km2) in 2011 to 34.9% (118 Km2) in 2021, especially thanks to Zostera noltei and Ruppia cirrhosa recolonization (Sfriso et al., 2017). The analysis of environmental parameters, carried out in 2021, highlighted the key role played by water transparency and salinity, which were strongly positively correlated to total number of taxa, to the presence of sensitive and crustose calcareous macroalgae of the genera Hydrolithon, Pneophyllum and Melobesia and to aquatic angiosperm cover, consisting mainly in Cymodocea nodosa and Zostera marina. In contrast, Chl-a, TSS, ammonium, nitrite, nitrate and silicate concentrations in water column, as well as moisture, phosphorus concentrations and Fines (fraction < 63 μm) in surface sediments, were strongly correlated to opportunistic species, especially Ulva rigida C. Agardh, Gracilariopsis longissimima (S. G. Gmelin) Steentoft et al., Agardhiella subulata (C. Agardh) Kraft et M. J. Wynne and Solieria filiformis (Kützing) P. W. Gabrielson. The correlation with the environmental parameters was extended also to 41 macroalgal taxa, i.e the most abundant taxa present in 20% of the considered stations. The obtained results evidence that a constant monitoring of these environmental parameters can be very useful in order to predict the most probable evolution of aquatic vegetation in TWS, especially with regard to the presence of sensitive species and aquatic angiosperms.