Factors determining the upper limit of giant kelp, Macrocystis pyrifera Agardh, along the Monterey Peninsula, central California, USA

Abiotic and biotic factors determining the upper (shallow or nearshore) limit of giant kelp, Macrocystis pyrifera Agardh, were examined along a wave exposure gradient on the Monterey Peninsula, central California, USA. Wave modeling, analysis of aerial photographs from 1986 to 1989 and SCUBA surveys from 1993 to 1995 indicated a significant positive relationship between wave intensity and depth of the upper limit of giant kelp; increased wave intensity resulted in the upper limit moving offshore into deeper water presumably due to direct removal of adult giant kelp plants by waves. Further, during periods of high wave intensity, plants with canopies were restricted to deeper water than those without canopies, suggesting that wave-induced giant kelp mortality was related to plant biomass (i.e. drag). Removal of giant kelp from shallow water ( less than or equal to 2.5 m depth) during periods of high wave intensity may have facilitated the development of dense algal turf assemblages by reducing light limitation; clearing experiments indicated that algal turf inhibited giant kelp recruitment at depths less than or equal to 2.5 m. Under extended periods of low wave intensity, however, giant kelp can establish populations in shallow water as indicated by the shallower depth of continuous giant kelp canopies with decreasing wave exposure. Thus, algal community structure in these shallow subtidal regions along the Monterey Peninsula appears to be determined by disturbance-mediated competition; with a lack of disturbance favoring giant kelp, disturbance favoring algal turf. These data support the hypothesis that the upper limit of giant kelp is controlled by an interaction between abiotic and biotic factors.