Effect of ecological recovery on macrophyte dominance and production in the Venice Lagoon.

In the last decade, the Venice Lagoon showed a significant environmental recovery that changed the assemblages of macroalgal and aquatic angiosperm dominant species and increased significantly the primary production. The decreasing of anthropogenic impacts, such as eutrophication and clam harvesting, favored a strong reduction of Ulvaceae, replaced by species with higher ecological value, and the recolonization of aquatic angiosperms. Consequently, hypo-anoxic conditions, once frequently occurring in the lagoon, have been considerably reduced and aquatic angiosperms have recolonized the area, covering 94.8 km2 in comparison to the 55.9 km2 recorded in 2003 (+70%). Cymodocea nodosa, Zostera marina and Zostera noltei expanded by 37.5%, 44.6% and 191%, respectively, with a significant increase in biomass and primary production. In late spring 2018, angiosperms showed a standing crop of ca. 372 ktonnes (+77%) and a net primary production of ca. 1189 ktonnes FW (+67%). In the meantime, Ruppia cirrhosa, which since the ‘80s had disappeared from the lagoon areas subjected to tidal expansion, but was still present in some fishing valleys, recolonized the bottoms of the northern lagoon with meadows of over 6 km2; this accounted for a standing crop and net primary production of 8.9 and 18.0 ktonnes, respectively. On the basis of surveys carried out in 2021, ecological conditions are still improving, and this is increasing both the biodiversity and the production of macroalgae and aquatic angiosperms