EFFECTS OF REMOVING SEA-URCHINS (STRONGYLOCENTROTUS-DROEBACHIENSIS) -STABILITY OF THE BARREN STATE AND SUCCESSION OF KELP FOREST RECOVERY IN THE EAST ATLANTIC

Stability properties of the barren state of a kelp forest-sea urchin s ystem were studied in northern Norway. The ability of the sea urchin S trongylocentrotus droebachiensis to maintain high population densities and recover from perturbations, and the succession of kelp forest rev egetation, were studied experimentally by reducing the sea urchin dens ity on a barren skerry. Additional information was obtained from commu nity changes following a natural, but patchy, sea urchin mortality tha t varied between sites. On the barren grounds, high sea urchin densiti es (30-50 per m(2)) is maintained by annual recruitment. Severe reduct ions of sea urchin densities initiated luxuriant kelp growth, while mo re moderate reductions allowed establishment of opportunistic algae (d uring spring and early summer), but no kelps. Succession of algal grow th, after the severe decline in sea urchin densities, followed a predi ctable pattern. At first the substrate was colonized by filamentous al gae, but within few weeks they were outcompeted by the fast growing ke lp Laminaria saccharina. After 3-4 years of the removal experiment, th e slower-growing, long-lived kelp L. hyperborea became increasingly do minant. Increased food availability after reduction in sea urchin dens ity led to increased individual growth of the remaining sea urchins. H owever, the population density did not increase, neither from recruitm ent nor immigration from adjacent areas with high sea urchin densities . Possibly, early establishment of a dense kelp stand, may represent a breakpoint in the ability of sea urchins to reestablish a barren stat e. The ability of L. saccharina quickly to invade and monopolize an ar ea may have both positive and negative effects on the succession towar ds the climax L. hyperborea kelp forest. Competitive interactions may slow the process, but development of a dense stand of L. saccharina wi ll also reduce grazing risk on scattered recruits of the more slowly g rowing L. hyperborea.