Macroalgae in the lagoon of Venice: long-term changes, alien taxa and environmental. Seventh European Phycological Congress

The first macroalgal studies in the lagoon of Venice date back to the end of the eighteenth century. They show only taxonomic data but without mentioning either the quantitative distribution of the species or any environmental relationship. Maps of the macroalgal distribution and density have been drawn since 1980 and studies on their relationship with the environment started a few years later. Over the past 40 years over three hundreds of papers have been produced deepening the close relationship between macroalgae, trophic status and ecological status. This knowledge was the basis for the development of the Macrophyte Quality Index (MaQI), a categorical index for the evaluation of the ecological status of the Mediterranean transitional water systems. MaQI is not based on the structure and morphology of macroalgae but on the correlations of macroalgae with numerous environmental parameters, nutrients and pollutants. This index can also be applied in the presence of negligible biomass or microscopic epiphytes, according to their ecological value. In the lagoon ca. 310 taxa are present and over 80 are sensitive species that colonize only environments of good-high ecological status. Furthermore, the Venice lagoon is a hotspot of non-indigenous species (NIS). At present ca. 30 NIS colonize the lagoon accounting for ca. 32% of the total macroalgal biomass. Among them 6 species account for 98.8% of the total NIS biomass and Gracilaria vermiculophylla (45.3%), Agardhiella subulata (25.1%) and Hypnea cervicornis (19.3%) are the most abundant species (89.7%). However, these NIS are not a concern for the environment. Some of them are rare species of high ecological value that increase biodiversity. Other NIS like G. vermiculophylla have a positive impact on the environment. This species grows under prohibitive conditions for the other macroalgae: high nutrient concentrations and very turbid waters, replacing Ulvaceae and avoiding anoxic crises.