Spreading of alien macroalgae in the Venice Lagoon, the Italian hotspot of nonindigenous species: biodiversity and standing crop.

Alien macroalgae are a constant concern for coastal areas, especially for the transitional systems of the northern Adriatic Sea. A revision of the taxa in the Venice Lagoon, the Italian hotspot of non-indigenous species (NIS), shows that, currently, the number of algal introductions is 31, and this number is growing steadily. On the basis of macrophyte distribution recorded during the last five years we estimated that the total NIS standing crop is ca. 147 ktonnes, i.e. 32% of the total macroalgal standing crop measured in late spring 2014 (ca. 456 ktonnes). The most abundant species are the invasive Gracilaria vermiculophylla (ca.66 ktonnes), Agardhiella subulata (37 ktonnes) and Hypnea flexicaulis (28 ktonnes). These species grow mainly free-floating and colonise mostly the soft substrata of the lagoon. Other two invasive species, which grew attached to hard substrata are Sargassum muticum and Undaria pinnatifida, but they showed a biomass significantly lower. Recent studies on Gracilaria vermiculophylla showed that, in turbid and confined areas rich in nutrients, this species is able to replace all the other species. Contrary to what it is believed, its presence has a positive effect on the environment because, in confined areas replace Ulvaceae, avoiding or reducing a rapid biomass collapse and the triggering of hypo-anoxic crises. In addition, the presence of NIS increases the biodiversity of the lagoon because the most abundant species have a seasonal distribution only.