The Effects of Canopy Shadings on Algal Recruitment and Growth in a Giant Kelp Forest

The subtidal (10—15 m) assemblage in the relatively sheltered giant kelp forest at Stillwater Cove in Carmel Bay, California, consists of perennial species forming three major vertical layers: a Macrocystis pyrifera surface canopy, a dense subsurface canopy of another kelp, Pterygophora californica, and an understory of articulated and encrusting coralline algae. The kelp canopies alone or in combination can reduce bottom light to <30% (usually <1%) of surface influx. The effects of light reduction by these vegetation layers on algal recruitment and subsequent growth were determined by removing various combinations of canopies over a 2—yr period, and following subsequent changes relative to appropriate controls. Removing both M. pyrifera and P. californica canopies resulted in moderate recruitment of these species as well as of the annual brown alga Desmarestia ligulata var. ligulata. None of these algae recruited into control areas where one or both canopies were left intact. Highest brown and red algal recruitment occurred when both kelp canopies plus understory coralline branches were removed. Removal of the latter alone had no significant effect. The time of year when algal canopies were removed had little effect on the composition of subsequent algal colonization, as the recruitment of noncalcareous species occurred primarily during a short period in the spring. These results indicate that the relatively low levels of both physical and biological disturbance in Stillwater Cove allow the establishment of a few perennial algal species that inhibit their own recruitment, as well as invasion of other species, by shading. This contrasts with nearby kelp forests subjected to greater and more frequent disturbance, and characterized by a diverse assemblage of annual algal species.