Annual growth and environmental relationships of the invasive species Sargassum muticum and Undaria pinnatifida in the lagoon of Venice

• Sargassum muticum and Undaria pinnatifida annual growth.
• Role of environmental parameters in macroalgal growth.
• Annual changes of tissue and environmental nutrient concentrations.
• Venice Lagoon.

The growth and autoecology of two alien invasive species: Sargassum muticum and Undaria pinnatifida spreading in the Venice Lagoon were studied monthly, during one year, in two sites of different depth. S. muticum was present year-round and reached its largest size (485 cm) and maximum growth (8.33 cm d−1) at the deepest station. U. pinnatifida was present only from November to May, reaching the highest size (130 cm) in March–April in the shallow station with growth peaks of 2.32 cm d−1. The growth of both species was mainly regulated by water temperature, nutrient concentration, especially nitrogen, and water turbidity. The study highlights the different ecological role already observed for the two species: U. pinnatifida prefers eutrophic areas and is not present along the sea-coastline. Its total standing crop does not exceed 0.2 ktonnes fwt for all the Venice Lagoon. Conversely, S. muticum colonizes areas with a lower eutrophication level, such as the lagoon inlets, reaching a total lagoon standing crop of 4–6 ktonnes fwt.