Sea urchin cavitation of giant kelp ( Macrocystis pyrifera C. Agardh) holdfasts and its effects on kelp mortality across a large California forest

Abstract

Sea urchins, Strongylocentrotus franciscanus (A. Agassiz) and especially S. purpuratus (Stimpson) sheltering in holdfasts of giant kelp, Macrocystis pyrifera, feed on haptera, eventually creating cavitation damage that leads to structural failure of the holdfast when the plants are stressed by large waves. Periodically giant kelp plants on permanent transects in a large Southern California forest were categorized for their degree of urchin infestation and cavitation damage, and subsequent survival followed for 5 yr. Plants with a high degree of urchin damage had significantly higher rates of mortality than plants with little damage during several assessment periods. There was a decreasing gradient in the degree of urchin damage and importance of cavitation from the deep (18 m), outer edge of the Point Loma forest, through the center (15 m), to the inner (12 m) edge of the forest which paralleled urchin abundance and recruitment rates. This gradient acts to reduce the impact of the gradient of giant kelp mortality caused by storms, which is much greater in shallow water and decreases seaward.