Algal succession in a Macrocystis pyrifera forest

Algal succession within a subtidal forest of the giant kelp Macrocystis pyrifera was studied by following colonization and community development on concrete blocks fastened to the bottom. Sets of blocks were placed in the bed at 3-month intervals. Subsequent algal development on each set was followed for over a year. All macroscopic species attached to the substrata were noted, and the number and lenght of basal branches determined every 1 to 3 months. Colonizing plants fell into 3 categories: rapid-growing ephemerals, and rapid and slow-growing perennials. Ephemerals such as Giffordia (Ectocarpus) mitchellae, Colpomenia peregrina, and diatom films generally produced and initial bloom on the blocks but were gradually replaced by perennials (articulated corallines, Rhodymenia spp., Gigartina spp.) characteristic of the mature kelp community. These stages, rather than representing “ecological” succession, seemed to reflect differences in growth rate and success in interspecific competition for space and light. Colonization on the blocks varied with season, indicating that most species have either a spring-summer or fall-winter period of maximum reproduction. M. pyrifera sporophyte colonization was greatest in spring. During community development, algal diversity (H), number of species (s) and evenness (J) all reached a peak within 100 to 200 days regardless of the time the blocks were started. Diversity and number of species then fell as ephemeral species disappeared. These species were apparently unable to compete with perennials and, once gone, did not recolonize. Evenness remained high.