Along the central California coast where the distributions of these two species overlap, giant kelp outcompetes bull kelp for light. Giant kelp dominates areas of relatively low water motion, and is dominant in years with relatively calm sea conditions. Bull kelp is more tolerant of high water motion and dominates more exposed areas (McLean 1962, Foster 1982, Harrold et al. 1988). The shallow areas inshore of these kelp forests are often characterized by canopies of the feather boa kelp Egregia menziesii, the intertidal giant kelp Macrocystis integrifolia, and the Fucalean alga Cystoseira osmundacea (Foster and Schiel 1985).
Foster (1982) observed that at Greyhound Rock, 24 km North of Santa Cruz, kelp canopies become sparse due to the combined effects of unsuitable substrate and increased water motion. This may help explain the lack of continuous kelp forests extending from Año Nuevo to the northern end of the MBNMS at Rocky Point.
Although individual giant kelp plants can live up to seven years (Rosenthal et al. 1974), plants in central California may be shorter lived because they are removed during periods of high water motion associated with winter storms (McLean 1962; Foster 1982; Reed and Foster 1984; Harrold et al. 1988). During these periods, giant kelp survivorship is positively correlated with the hardness of the substrate to which the plants are attached (Foster and Schiel 1985). Another major source of kelp mortality along central California has been grazing by sea urchins (Strongylocentrotus spp.) (Pearse and Hines 1979, Watanabe and Harrold 1991). The effect of urchins however, has decreased due to predation by the sea otter Enhydra lutris (McLean 1962, Kenner 1992, Watanabe and Harrold 1991).
Although much is known about growth and survivorship of adult kelp sporophytes, relatively little is known about the ecology of their microscopic stages. These stages are probably highly vulnerable to grazing (Leonard 1994) and sedimentation (Devinny and Volse 1978, Deysher and Dean 1986) such as that from sewage discharges, which may thus be important in determining the distribution of kelp forests within the sanctuary (see also Meistrell and Montagne 1992). Studies on microscopic stages of giant kelp suggest they are also sensitive to the toxins associated with municipal and industrial waste discharges. For example, growth and fertility are reduced when gametophytes are exposed to elevated copper concentrations in discharge effluent (Anderson et al. 1990; see also Foster et al. 1983 for a review).
Next - Section II. Algal Assemblages Associated with Kelp Forests
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