Vulnerability of marine habitats to the invasive green alga Caulerpa racemosa var. cylindracea within a marine protected area.

The relative vulnerability of various habitat types to Caulerpa racemosa var. cylindracea invasion was investigated in the National Marine Park of Zakynthos (Ionian Sea, Greece). The density of C. racemosa fronds was modelled with generalized additive models for location, scale and shape (GAMLSS), based on an information theory approach. The species was present in as much as 33% of 748 randomly placed quadrats, which documents its aggressive establishment in the area. The probability of presence of the alga within randomly placed 20 x 20 cm quadrats was 83% on 'matte morte' (zones of fibrous remnants of a former Posidonia oceanica bed), 69% on rocky bottoms, 86% along the margins of P. oceanica meadows, 10% on sandy/muddy substrates, and 6% within P. oceanica meadows. The high frond density on 'matte morte' and rocky bottoms indicates their high vulnerability. The lowest frond density was observed within P. oceanica meadows. However, on the margins of P. oceanica meadows and within gaps in fragmented meadows relative high C. racemosa densities were observed. Such gaps within meadows represent spots of high vulnerability to C. racemosa invasion.