G. Bellemo

Distribution of introduced Japanese macroalgae Undaria pinnatifida, Sargassum muticum (Phaeophyta) and Antithamnion Pectinatum (Rhodophyta) in the Lagoon of Venice

The present distribution of invasive seaweeds Undaria pinnatifida (Phaeophyta, Laminariales) and Sargassum muticum (Phaeophyta, Fucales) in the Lagoon of Venice is updated in this work. The distribution of Antithamnion pectinatum (Rhodophyta, Ceramiales) is presented for the first time. All these species have quickly colonized the hard substrata competing with indigenous species. Competition acts differently in the two species, U. pinnatifida competing for the substratum and S. muticum for light. On tidal flats composed of silt and sand, the growth of two brown seaweeds is restricted owing to the lack of a solid substrata. While Undaria pinnatifida and Sargassum muticum have colonized only some areas of the lagoon in a short vertical range (from 0 to -3 m), Antithamnion pectinatum is present on the three portmouths, along the inner shores of the city of Venice, and in the lagoon islands with extensive vertical distribution (from +0.2 to -8 m).

The introduced alga Undaria pinnatifida (Laminariales, Alariaceae) in the lagoon of Venice

Since its appearance in 1992 in the lagoon of Venice, the brown algae Undaria pinnatifida (kelp) has gradually expanded along the banks of canals both at Chioggia and Venice, becoming the dominant species in the local algal community chiefly from February to July. In Chioggia Island, where another brown seaweed (Sargassum muticum) is present since 1992, the spreading of Undaria reaches a plateau. In Venice Island, instead, the colonisation process is still in progress. During 1999, the kelp has colonised the main canals (e.g. The Grand Canal) and, subsequently, the small inner ones. In order to evaluate the dynamics of substrate re-colonisation by Undaria, two mechanical eradications were carried out during (March) and after the fertile period (July) of the algae. Such experimental manipulations provided evidence of the fast re-colonisation potential of the algae mainly attributable to its efficient reproductive system. Eradication made during the fertile period, in fact, permitted the kelp development during the following year, while re-colonisation has started 2 years later where eradication was performed after the reproductive period. A significant decrease in the surface covered by other species has been observed both in shallow (Ulva rigida, Enteromorpha spp., Antithamnion pectinatum, Chondracanthus acicularis) and deeper areas (Rhodymenia ardissonei) during the period of maximum development of U. pinnatifida. On the basis of the results of this study, the following conclusions can be drawn: (1) the alga U. pinnatifida is continuously expanding in lagoon environments of Venice; (2) in order to limit its spreading, mechanical eradications would be done on a large spatial scale and before the zoospores release; (3) there is suggestive evidence of competition between Undaria and the remaining indigenous algae.