California Sea Grant College Program

The high productivity and diversity of giant kelp forests have long attracted the attention of biologists. After observing the kelp forests of Tierra del Fuego, Charles Darwin (1860, p. 227-229) wrote: "There is one marine production, which from its importance is worthy of a particular history. It is the kelp, or Macrocystis pyrifera…The number of living creatures of all Orders, whose existence intimately depends on the kelp, is wonderful…I can only compare these great aquatic forests of the southern hemisphere, with the terrestrial ones in the intertropical regions. Yet if in any country a forest was destroyed, I do not believe nearly so many species of animals would perish as would here, from the destruction of the kelp."

Along the coast of California, forests of giant kelp also harbor rich and complex communities of marine plants and animals. The environmental conditions and species composition of these forests vary, but they all have one factor in common: the dominating presence of the giant kelp, Macrocystis sp. In what ways are the organisms associated with this plant influenced by it and dependent on it?

Macrocystis plays a major role in determining the physical structure of the kelp forest community. It not only substantially increases the total substrate area (Clendenning, 1960, in North, 1971a), but also extends that substrate vertically through the water column to the sea surface. The morphological heterogeneity of the plant itself provides a diversity of habitats (North, 197la; Pearse and Gerard, in press). The influence of Macrocystis on the physical structure of the forest is comparable to that of the trees in a terrestrial forest and of the hermatypic corals in a tropical reef community.

Macrocystis also influences the trophic structure of the forest community as the dominant primary producer, outweighing and outproducing the other benthic algae and phytoplankton (Clendenning, 1971; North, 197la). The magnitude and fate of the kelp production and its role as a food base within the forest community are the major concerns of this thesis.

In the first chapter, I investigate the dynamics of the attached kelp population, and examine the magnitude and mechanisms of drift kelp production and its importance, relative to grazing and detritus formation, as a pathway of primary production within and out of the kelp forest. The formation of drift kelp influences the availability of kelp production to and its utilization by members of the forest community. This trophic pathway is discussed in the second chapter, in which I consider benthic drift plant material as a resource within the forest. Finally, in the third chapter, I test my hypotheses on the role of benthic drift algae and seagrasses as a trophic pathway by study­ing the feeding habits of the drift consumer, Patiria miniata. The dependence of this asteroid on the drift plant resource may provide insight into the dependence of other kelp forest inhabitants on the productivity of Macrocystis.