Scale of impact determines early post-disturbance assemblage structure in subtidal Fucus beds in the Baltic Sea (Bornholm, Denmark).

Disturbances occasionally create gaps of varying sizes in the canopy cover of macroalgae on temperate reefs. This study quantified the size distribution of naturally occurring gaps in a subtidal Fucus serratus canopy, and experimentally tested the hypotheses that (i) early post-disturbance abundance of dominant assemblage-formers in the canopy-dominated habitat will depend on the spatial extent of the impact, and (ii) the effects of disturbance on the abundance of dominant assemblage-formers at the edges of large impacted areas will be similar to the effects at small impacted areas. Gaps of up to 6?m diameter accounted for 17% of the habitat and, while small gaps were numerically dominant, all gap size-classes accounted for a similar proportion of habitat. Experimental disturbances (canopy clearing) ranging from 0.6 to 4.8?m diameter caused consistent scale-dependent divergence of assemblage structure after 21 days. Of particular interest was a dense cover (~75%) of filamentous algae in the centres of large (=1.2?m) clearings. Filamentous algae are widely known to have negative effects on Fucus recruitment and growth, and their rapid increase in abundance suggests that future recovery of the Fucus canopy could be adversely affected. The abundance of most taxa at the edges of large clearings was either indistinguishable from abundances in small clearings and intact canopy, or intermediate between abundances in these treatments and in the centres of large clearings. Juvenile Fucus were, however, more abundant around edges than anywhere else. These experimental outcomes support the model that scale-dependent divergence of assemblage structure is conveyed by an escape from processes associated with edges of the original habitat. I propose that, in the subtidal Fucus beds of Bornholm, recovery from large disturbances is restricted to encroachment from the edges because the adjacent canopy suppresses the growth of filamentous algae and thus facilitates regeneration and recruitment of Fucus . [ABSTRACT FROM AUTHOR]