Interactions beween grazers and algal canopies, an experimental approach

'. Interactions between macroalgae and grazers were investigated in manipulative field experiments on intertidal sheltered rocky shores on the Swedish west coast and on the Isle of Man. One grazer exclosure experiment was also made on semi-exposed shores on the Swedish west coast. On sheltered shores in Sweden, exclusion of periwinkles Littorina spp. did not affect the survival of germlings of the brown alga Ascophyllum nodosum in presence of adult canopy. Other grazers such as isopods and amphipods were suggested to be important. However, increased encaged numbers of L. littorea caused lower survival. In another experiment the germling survival was affected by complex interactions between Littorina spp. , adult canopy and germling density. Survival was low in absence of canopy and presence of Littorina spp. , but higher in absence of Littorina spp. Later the survival was lower in presence of Littorina spp. , irrespective of canopy treatment. Survival of germlings in high densities was lower in presence of canopy than in areas cleared of canopy, but only in presence of ephemeral green algae. A four-year experiment explored the effect of canopy removal and grazer exclosure on recruitment of A. nodosum juveniles and the community structure. Canopy removal transformed the assemblage for at least 31 months. Removal of both canopy and periwinkles increased ephemeral green algae after 3 months. After 18 months a canopy of Fucus spp. had developed in plots initially cleared of canopy, but A. nodosum juveniles were not affected. A similar experiment on the Isle of Man, manipulating also the red algal turf, canopy removal in interaction with intact turf, resulted in higher abundances of juvenile A. nodosum. Removal of both canopy and the limpet Patella vulgata resulted in much higher A. nodosum recruitment. Canopy removal in interaction with either turf or limpet removal gave a dense cover of ephemeral green algae after 5 months, and later a dense cover of Fucus spp. On semi-exposed rocky shores in Sweden, exclosure of Littorina spp. gave no recruitment of fucoids, the reason might be that the exclosure was only effective during the first months. The results emphasized that complex interactions control these communities. be found in numbers of 1000 to 4000 ind. m" in the relative shelter of barnacles and clefts in the rock (see Reid 1996 for a review). In sheltered areas it is found higher up in the supralittoral on smooth rocks, but high numbers are also found among shingle and in stony areas lacking A. nodosum (Stephenson & Stephenson 1972). Plots of the size of 50 by 50 cm were placed in the middle of the intertidal, and fences were used to exclude periwinkles. Two different semi-exposed areas around TMBL were used, and within these areas different shores with replication of plots were used. Time was included in the design, with one winter start, and one summer start of the experiments. The cover and development of different species were measured using 100 points in each plot. Fences used to exclude periwinkles were not very successful, and the plots were cleared of invading periwinkles at each time of monitoring.