Macrophyte assemblage associated with an invasive species exhibiting temporal variability in its development pattern

Many invasive species show temporal variations in abundances, such as seasonal cycles and medium to long term variations. The effects of invasive species on native assemblages have, however, mainly been investigated during the maximum abundance of the invasive species and rarely over an entire year. At the north-western limit of its distribution range in the Mediterranean Sea, the invasive marine macrophyte Caulerpa racemosa var. cylindracea shows high temporal variability in its development pattern with complete withdrawal in winter. The present study investigated the variation of the macrophyte assemblage associated with C. racemosa var. cylindracea in the bay of Marseilles (NW Mediterranean, France) over a period of one year. A comparison was made with a non-invaded assemblage at the same site. The results showed a relationship between the development of C. racemosa var. cylindracea and the structure of the macrophyte assemblage. When C. racemosa var. cylindracea was present, the associated assemblage showed significantly lower numbers of species, lower cover of native and introduced macrophytes and lower diversity (Shannon diversity index and Pielou's evenness) compared to the non-invaded assemblage. During the temporary winter withdrawal of C. racemosa var. cylindracea, an increase in the assemblage descriptors was noted; however, this period was probably not long enough to allow the recovery of a structured macrophyte assemblage comparable to the non-invaded assemblage. Other invasive macrophytes occur in the bay of Marseilles; their cover was, however, greatly reduced in the invaded assemblage, indicating strong competitive interactions between C. racemosa var. cylindracea and the other invasive species.