Response of the non-indigenous Caulerpa racemosa (Forsskal) J-Agardh to the native seagrass Posidonia oceanica (L.) Delile:: effect of density of shoots and orientation of edges of meadows

Caulerpa racemosa is a tropical green alga introduced into the Mediterranean as an immigrant from the Red Sea which has successfully fast-spread in the south-eastern and in the north-western part of the basin. C. racemosa occurs mostly in shallow but also in deep subtidal habitats colonising hard and soft substrata where turfs, erect algae and even seagrasses are present with the potential to profoundly alter indigenous communities. However, the extent to which biotic interactions influence the spread of the alga is not well known. In this study the effects of the presence of the native seagrass Posidonia oceanica on the non-indigenous alga Caulerpa racemosa are examined: a multifactorial experiment was designed to test (1) the importance of the seagrass canopy structure and (2) of orientation of seagrass edge on algal performance along the edge and inside the meadow of P. oceanica and (3) whether patterns of algal growth are consistent at different spatial scales (few centimetres to several metres). The aim of this study is to provide a basis for further experimental investigations of the factors and mechanisms affecting the performance of this alga in the Mediterranean. The results of this study indicated that where Caulerpa racemosa is at the edge of Posidonia oceanica meadow, the vertical growth of the alga (blade length) is sensitive to the combination of time, seagrass density and edge-meadow orientation, that the spread of the alga along the edge of the seagrass meadow is dependent on the characteristics of the area and that the growth of the alga inside the meadow is influenced by seagrass density. The findings of this study suggest that the susceptibility of the indigenous P. oceanica community to invasion of the introduced alga C. racemosa is related to the availability of sand habitat ground created, since low invasion of the very dense edges of the seagrass was observed compared to the less dense ones. (C) 2000 Elsevier Science BN. All rights reserved.