Effects of Posidonia oceanica canopy on size in a north-western Mediterranean bay

Abstract

Caulerpa taxifolia (Vahl) C. Agardh is a green tropical seaweed recently introduced in the Mediterranean. Since 1984, when for the first time it was recorded along the French Riviera, it has rapidly spread into a wide range of sublittoral habitats and has contributed to the decay and regression of the seagrasses Cymodocea nodosa (Ucria) Ascherson and Posidonia oceanica (L.) Delile. Descriptive field studies have indicated a strong habitat effect on C. taxifolia performance and suggested a positive influence of seagrasses, particularly of P. oceanica , on both blade density and size of the alga. Here we present two experimental investigations that have been carried out in situ to highlight mechanisms that regulate this kind of interaction. The aim of this study was to elucidate factors inducing the performance of this alga in the Posidonia oceanica habitat, to provide a basis to detect determinants of success for the invader non-indigenous species, and to predict vulnerability to invasion for different conditions of the seagrass beds. In the first experiment Posidonia oceanica shoot density and canopy height were manipulated along the edge of the meadow at 2 and 10 m depth and the response of Caulerpa taxifolia was assessed after four different elapsed times. The second experiment, designed to investigate the mechanism responsible for the canopy influence on blade size, employed mimic plants made of transparent plastic strips able to protect but not shade the alga. Because of the epiphyte growth which progressively attenuated the transparency effect of the treatment, only short-term results could be obtained. Overall results have indicated that the positive effect of Posidonia oceanica on Caulerpa taxifolia is greater at the deepest edge of the seagrass stand where the shoot density is lower and have suggested contrasting effects of the canopy on the alga. This has been partially explained by the fact that at the deeper site algal blades were greater where they occurred at the edge of mimic transparent plants indicating that the protection has a cost. Therefore, we predict that dense P. oceanica meadows are likely to be less vulnerable to seaweed invasion, whereas sparse meadows represent an optimal compromise of protection and shading.