Résumé
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.