The Mediterranean introduced seagrass Halophila stipulacea in eastern Sicily (Italy): temporal variations of the associated algal assemblage

Since the opening of the Suez Canal in 1869, a number of marine species haveentered the Mediterranean Sea from the Red Sea. The seagrass Halophila stipulacea was amongthese introduced species. In the course of the past century, H. stipulacea developed extensivemeadows in the eastern Mediterranean basin and is now extending its distribution into the westernbasin. As little is known of their vegetal assemblage, a temporal study of a meadow was carriedout off the eastern coast of Sicily throughout a year cycle. The associated algal communitywas dominated by epiphytes followed by rhizophytic species. Species diversity showed significantvariations over time with a maximum in October and a minimum in April. Changes inspecies diversity and community structure were positively correlated with the yearly cycle ofs e a g r a s s ' c o v e r. The seagrass bed was also populated by the introduced rhizophytic green algaCaulerpa racemosa, representing the second dominant vegetal species after H. stipulacea. Acomparison between the associated algal assemblages of this H. stipulacea meadow and twoother contiguous ones of seagrass Posidonia oceanica and Cymodocea nodosa, respectively,showed significant differences in species composition.