Effects of small-scale disturbances of canopy and grazing on intertidal assemblages on the Swedish west coast

The effects of small-scale disturbances (80×30-cm plots) of canopy and grazers on intertidal assemblages were investigated in this 4-year experiment on sheltered rocky shores on the Swedish west coast. Canopy disturbances due to ice scouring were mimicked by removal of adult plants of the seaweed Ascophyllum nodosum (L.) Le Joli. Density of the main epilithic grazing gastropods, Littorina spp., was lowered by exclosure and handpicking. Based on earlier experiments in other areas, the general hypothesis was that canopy removal and grazer exclosure, alone or in combination, should increase the recruitment of A. nodosum or other fucoid juveniles, and change the structure of the understorey assemblage.There was an effect of canopy removal on the development of this assemblage, lasting for more than 31 months. Both increased and decreased abundances of species were found as short-term effects, but there was also a longer-term effect with increased abundance. Grazer exclosure was only effective in combination with canopy removal, causing a short-term increase in ephemeral green algae. Short-term effects of canopy removal were also the increase in recruitment of Semibalanus balanoides (Linnaeus) and the decrease of the red alga Hildenbrandia rubra (Sommerfelt) Meneghini. Fast recruitment and growth of fucoid species (Fucus serratus L. and F. vesiculosus L.) restored the canopy and conditions of the understorey within 18 months. Thus, the canopy removal changed the physical conditions for the understorey, making it possible for other species to coexist in this community. Surprisingly, no effect of canopy removal or grazer exclusion was found on the recruitment of juvenile A. nodosum, neither by canopy removal nor grazer exclosure. The lack of such effects might be due to the early mortality caused by other grazers (small, mobile crustaceans), or to the low density of periwinkles on these shores. However, despite the patchy and generally low recruitment of A. nodosum juveniles, observations suggested that the cover of A. nodosum in manipulated patches would return to initial levels, either by recruitment or regrowth of small holdfasts and from growth of edge plants.