The role of disturbance in promoting the spread of the invasive seaweed Caulerpa racemosa in seagrass meadows

Human disturbances, such as anchoring and dredging, can cause physical removal of seagrass rhizomes and shoots, leading to the fragmentation of meadows. The introduced green alga, Caulerpa racemosa, is widely spread in the North-West Mediterranean and, although it can establish in both degraded and pristine environments, its ability to become a dominant component of macroalgal assemblages seems greater in the former. The aim of this study was to estimate whether the spread of C. racemosa depends on the intensity of disturbance to the canopy structure of Posidonia oceanica. A field experiment was started in July 2010 when habitat complexity of a P. oceanica meadow was manipulated to simulate mechanical disturbances of different intensity: rhizome damage (High disturbance intensity = H), leaf removal (Low disturbance intensity = L), and undisturbed (Control = C). Disturbance was applied within plots of different size (40 × 40 cm and 80 × 80 cm), both inside and at the edge of the P. oceanica meadow, according to an orthogonal multifactorial design. In November 2011 (16 months after the start of the experiment), no C. racemosa was found inside the seagrass meadow, while, at the edge, the cover of the seaweed was dependent on disturbance intensity, being greater where the rhizomes had been damaged (H) than in leaf removal (L) or undisturbed (C) plots. The results of this study indicate that physical disturbance at the margin of seagrass meadows can promote the spread of C. racemosa.