Coexistence of Ulva rigida C. Agardh and Ulva laetevirens Areschoug (Ulvales, Chlorophyta) in Venice Lagoon and other Italian transitional and marine environments.

There is confusion about the identity of laminar Ulva thalli in Italian and Mediterranean transitional systems and on the open coasts. Although some authors report the presence of both U. rigida and U. laetevirens, systematic revisions for the Adriatic and Mediterranean Seas recognise only the presence of U. laetevirens. This paper uses anatomical and environmental analyses to provide evidence for the presence of both species. The only diagnostic character that distinguishes the two is cell shape in cross section of rhizoidal and basal regions. Therefore, identification of floating thalli in these regions is not possible. The species grow and coexist in the same environments and on the same substrata, but analysis of environmental data suggests that U. rigida occurs preferentially in eutrophic environments, especially lagoon areas rich in phosphate, ammonium and phytoplankton, whereas U. laetevirens is more abundant in areas where nutrient levels are low. © 2010 by Walter de Gruyter Berlin New York.