MEPS 313:85-94 (2006)  -  doi:10.3354/meps313085

ABSTRACT: The invasibility of a low intertidal macroalgal assemblage was experimentally tested from March 2003 to April 2004 at 1 locality in northern Spain. It was hypothesised that a community becomes more susceptible to invasion when there is an increase in the amount of key resources. A bifactorial (‘nutrient supply’ and ‘macroalgal biomass removed’) orthogonal experiment was designed with 3 levels in each factor (high, medium and control). Fertile plants of Sargassum muticum (Yendo) Fensholt were transplanted to each plot to simulate the arrival of an invader. The invasibility of the assemblage was quantified in the pre- (density of recruits) and post-settlement (percentage cover, size and density of S. muticum at the end of the experiment) phases of S. muticum’s life cycle. Results supported the initial hypothesis. Both space availability and nutrient enrichment facilitated the establishment and spread of S. muticum in the experimental plots. Established S. muticum plants grew faster in enriched plots than in controls. Furthermore, different successional assemblages played different roles in resisting invasion as S. muticum’s life cycle progressed. In the initial stage of the invasion, the Bifurcaria bifurcata canopy inhibited recruitment by S. muticum, whereas understory species did not have a significant effect on invasion success. In contrast, an increased survivorship of S. muticum beneath the canopy of B. bifurcata was observed in those plots where S. muticum had successfully recruited. This study shows that the invasibility of this low intertidal assemblage is mediated by a complex interaction of several resources acting at different stages during S. muticum’s invasion.

KEY WORDS: Sargassum muticum · Invasibility · Marine invasion · Nutrients · Disturbance · Bifurcaria bifurcata · Macroalgal assemblage