MEPS 305:31-40 (2005)  -  doi:10.3354/meps305031

ABSTRACT: The long-living species Ascophyllum nodosum dominates the mid-shore of sheltered rocky sites in NW Europe, forming near-monospecific stands. We aimed to determine the effects of small-scale disturbance on such shores, through manipulative experiments over a long period (almost 7 yr). We tested the general hypothesis that small-scale disturbance allows co-existence of Fucus canopy (F. vesiculosus, F. serratus) and promotes recruitment of Ascophyllum itself. Two factorial experiments were established to determine the interactive effects of the Ascophyllum canopy with: (1) the red algal turf which covers approximately 90% of the substratum at the experimental sites, and (2) the dominant grazer, the limpet Patella vulgata, which maintains small patches of open substratum free from turf. Disturbance to plots 25 × 25 cm resulted in significant changes to the understorey community. Removal of limpets led to high levels of Ascophyllum and Fucus recruitment, irrespective of the presence of the canopy. In the presence of limpets recruitment of fucoids was negligible. In contrast, removal of turf alone led to high Fucus spp. but not Ascophyllum recruitment. Loss of the canopy and a consequent degradation of the turf was required to promote Ascophyllum recruitment. Despite high recruitment in some treatments beneath an intact canopy, low growth rates in shade prevented development of mature plants. Loss of the overlying Ascophyllum canopy was followed by development of a mixed F. serratus/F. vesiculosus canopy that was still present nearly 7 yr after manipulation. Ascophyllum recruits, while showing enhanced growth in the absence of the canopy, were too slow-growing to form a canopy.

KEY WORDS: Ascophyllum nodosum · Fucus spp. · Canopy disturbance · Recruitment · Interspecific competition · Rocky shores