MEPS 188:81-92 (1999)  -  doi:10.3354/meps188081

Direct and indirect effects of a macroalgal canopy and limpet grazing in structuring a sheltered inter-tidal community

Stuart R. Jenkins1,*, Stephen J. Hawkins2, Trevor A. Norton1

1Port Erin Marine Laboratory, University of Liverpool, Port Erin IM9 6JA, Isle of Man, UK
2Division of Biodiversity and Ecology, University of Southampton, Bassett Crescent East, Southampton SO16 7PX, England, UK

ABSTRACT: The dominant components of the mid-shore community of 4 sheltered, rocky shores in the south of the Isle of Man, UK, were surveyed. A 2-way factorial experiment to investigate the community structuring roles of Ascophyllum nodosum (hereafter Ascophyllum) canopies and Patella vulgata grazing was undertaken at 1 site and monitored over a period of 6 yr. Removal of the canopy had a marked impact on the understorey community, with both direct and indirect effects. In contrast, the effect of limpet removal was limited, owing to the restriction of this grazer to small patches of bare substrate within a turf of red algae. The Ascophyllum canopy directly facilitated the presence of the red algal turf in the mid-shore; canopy removal resulted in bleaching and death of turf species with a consequent loss of entrapped silt. This degradation of the turf resulted in an increase in the area grazed by limpets and a subsequent increase in limpet recruitment, proportional to the increased area of bare substratum. Thus, the Ascophyllum canopy indirectly limits the population of P. vulgata by facilitating the growth of a red algal turf. Eighteen months after Ascophyllum removal, a mixed canopy of Fucus vesiculosus and Fucus serratus developed. This acted in a similar manner to the original canopy, providing shade for turf species, which resulted in restoration of the balance between algal turf and limpet grazing. Despite this, the red algal turf had not fully recovered 5 yr after it was originally bleached, illustrating the long-term effects of Ascophyllum canopy loss on this community. Removal of the canopy also resulted in high levels of Ascophyllum recruitment, but the slow growth rate of these juvenile plants meant that nearly 6 yr after canopy removal, an Ascophyllum canopy had still not developed. Nonetheless, we predict that Ascophyllum will eventually outcompete the established Fucus spp. canopy. Detailed examination of the distribution of Ascophyllum juveniles suggests that the low density of juveniles in the natural population is due to a lack of substratum free from both limpets and space-occupying turf, rather than a direct effect of the canopy.

KEY WORDS: Ascophyllum nodosum · Patella vulgata · Community structure · Grazing · Indirect effects · Canopy algae

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