SEA-URCHIN CAVITATION OF GIANT-KELP (MACROCYSTIS-PYRIFERA AGARDH,C.) HOLDFASTS AND ITS EFFECTS ON KELP MORTALITY ACROSS A LARGE CALIFORNIA FOREST

Sea urchins, Strongylocentrotus franciscanus (A. Agassiz) and especial ly S. purpuratus (Stimpson) sheltering in holdfasts of giant kelp, Mac rocystis pyrifera, feed on haptera, eventually creating cavitation dam age that leads to structural failure of the holdfast when the plants a re stressed by large waves. Periodically giant kelp plants on permanen t transects in a large Southern California forest were categorized for their degree of urchin infestation and cavitation damage, and subsequ ent survival followed for 5 yr. Plants with a high degree of urchin da mage had significantly higher rates of mortality than plants with litt le damage during several assessment periods. There was a decreasing gr adient in the degree of urchin damage and importance of cavitation fro m the deep (18 m), outer edge of the Point Loma forest, through the ce nter (15 m), to the inner (12 m) edge of the forest which paralleled u rchin abundance and recruitment rates. This gradient acts to reduce th e impact of the gradient of giant kelp mortality caused by storms, whi ch is much greater in shallow water and decreases seaward.