Impact of the invasive green alga Caulerpa racemosa var. cylindracea on the epiphytic macroalgal assemblage of Posidonia oceanica seagrass rhizomes in the Adriatic Sea

Changes of both macroalgal diversity and vegetational characteristics of the epiphytic macroalgal assemblage on Posidonia oceanica rhizomes of a meadow occurring at Marinkovac Island (central Adriatic Sea), following the invasion of Caulerpa racemosa var. cylindracea, are reported. The study was based on the comparison between epiphytic macroalgae collected on rhizomes in an invaded and a reference (non-invaded) P. oceanica meadow. Samples were collected in autumn 2002, during the annual maximum development of the C racemosa var. cylindracea in order to assess its maximum impact. A significant reduction in both the number of epiphytic algal taxa and their cover was observed. A total of 71 epiphytic macroalgal taxa were found on the P. oceanica rhizomes within the reference area unaffected by C racemosa var. cylindracea (Rhodophyta 55, Ochrophyta 5, Chlorophyta 11), compared with 20 taxa on rhizomes in the invaded area (Rhodophyta 18, Ochrophyta 1, Chlorophyta 1). The number of Rhodophyta, Ochrophyta and Chlorophyta were 3.1, 5 and I I times lower, respectively, while the mean number of taxa per rhizome decreased by 6 times. The greatest decrease of the mean total percentage cover of macrolagal epiphytes (mRt%) in the invaded area was in Ochrophyta, ecological group D and calcareous encrusting algae, and was by 11.6, 17.4 and 13.3 times, respectively. The results give additional information on the extremely invasive behaviour of C. racemosa var. cylindracea in the Mediterranean Sea.