Interactions between canopy forming algae in the eulittoral zone of sheltered rocky shores on the Isle of Man Stuart R. Jenkins, Trevor A. Norton and Stephen J. Hawkins* Port Erin Marine Laboratory, University of Liverpool, Port Erin, Isle of Man, IM9 6JA. *Division of Biodiversity and Ecology, University of Southampton, Biomedical Sciences Building, Bassett Crescent East, Southampton, SO16 7PX The distribution and abundance of Ascophyllum nodosum, Fucus serratus and F. vesiculosus were described at four sheltered, rocky shores in the south of the Isle of Man. Canopy removal experiments were performed at mid tide level of one sheltered, canopy dominated shore to investigate the interactions between the dominant canopy alga, Ascophyllum nodosum and the competitively inferior canopy species of Fucus serratus and F. vesiculosus. Ascophyllum was removed from replicated plots, 2  2 m in size, in both winter and summer; the early growth and survival of fucoids in the presence and absence of the Ascophyllum canopy were monitored and the eventual development of a new canopy described. Juveniles of F. serratus origin- ally present beneath the undisturbed canopy of Ascophyllum died following canopy removal but new recruitment resulted in some canopy development, principally in the winter experiment. Fucus vesiculosus, despite being completely excluded from the Ascophyllum zone of all four shores described, dominated the canopy removal plots of both winter and summer experiments. The Ascophyllum canopy did not recover over a ¢ve year period of observation, although a considerable increase in the abundance of Ascophyllum juveniles occurred. INTRODUCTION Sheltered rocky shores of north-west Europe are domi- nated by canopy forming fucoid algae. In very sheltered areas a continuous cover of canopy from the high shore to the sublittoral zone may occur. A dominant feature of such shores is the often extensive mid shore zone of Ascophyllum nodosum (L.) Le Jolis which is very stable over time (Lewis, 1964). David (1943) considered that in the mid shore of sheltered areas the three fucoid species, A. nodosum, Fucus vesiculosus L. and Fucus serratus L. compete for space. The abundance and zonation of these three species shows great variability (Lewis, 1964), although some general patterns are consistent. Fucus serratus generally forms a distinct zone low on the shore between the mid shore Ascophyllum and the laminarians of the sublittoral fringe. The vertical position of F. vesiculosus may vary according to the degree of shelter, either forming a mixed popula- tion with Ascophyllum (Lewis, 1964; Hawkins et al., 1992) or in more sheltered areas being restricted to a narrow fringe at the top of the Ascophyllum belt. Occasionally a belt beneath Ascophyllum can occur (Lewis, 1964). On Manx sheltered shores the mid shore is dominated by Ascophyllum. Within this apparently monospeci¢c canopy, F. serratus is abundant especially in the low and mid parts of the Ascophyllum zone. Fucus vesiculosus is scarce and restricted to the upper part of the zone. The community ecology of the Ascophyllum zone of shel- tered sites has been relatively neglected. A number of studies have investigated the potential of Ascophyllum for regrowth or recolonization in harvested or experimen- tally denuded areas, both in Europe (Printz, 1956) and North America (Keser et al., 1981; Keser & Larson, 1984; Sharp, 1986). Studies examining the interaction between Ascophyllum and other canopy species are rare and have been mainly concerned with understanding the pattern of vertical zonation (e.g. Hawkins & Hartnoll, 1985). Many authors have remarked upon the surprisingly low levels of Ascophyllum juveniles which recruit into mature stands (Oltmanns, 1889; David, 1943; Knight & Parke, 1950; Printz, 1956; Baardseth, 1970; Vadas et al., 1990), although a recent study has reported densities as high as 46 juve- niles m 72 in the upper part of the Ascophyllum zone (Aberg & Pavia, 1997). However, recruitment of Ascophyllum is certainly low in relation to the large invest- ment placed into reproductive biomass ( Josselyn & Mathieson, 1978; Cousens, 1986; Aberg, 1996) and it seems survival and growth of established plants is more important for population growth than sexual reproduc- tion (Aberg, 1992). These factors combined with the slow growth rate of Ascophyllum (Schonbeck & Norton, 1980) make investigations into competitive interactions in this zone di¤cult. Competition among marine macroalgae has been demonstrated or suggested to be important in deter- mining the distribution and abundance of species in both intertidal (Dayton, 1975; Hawkins & Hartnoll, 1985; Chapman, 1990), and subtidal communities (Dayton et al., 1984; Reed & Foster, 1984). Although Ascophyllum is clearly dominant on sheltered shores, it grows slowly compared to Fucus species with which it competes (Schonbeck & Norton, 1980). Combined with low recruit- ment, recolonization of disturbed areas is very slow. The aim of this work was to investigate the interactions between Ascophyllum and subdominant Fucus species, J. Mar. Biol. Ass. U.K. (1999), 79, 341^349 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom (1999)