features of the environment (see Chapters 



2 and 5), plants may also change these 



features as they grow and spread 



vertically through the water column. 



Following Foster (1975a), we have 

 divided the kelp forest vegetation 

 functionally into four layers above the 

 primary substratum (Figures 3 and 6): 

 encrusting species, filamentous and 

 foliose species, understory Laminariales 

 and Fucales, and the species forming 

 surface canopies. Each of these levels 

 will have an effect on the recruitment, 

 growth and survival of species below, and 

 because all start life on the bottom, on 

 themselves. Complete descriptions of 

 California seaweeds, including geography 

 and depth distributions, can be found in 

 the taxonomic work by Abbott and 

 Hollenberg (1976), and the biology and 

 natural history of the most abundant 

 species in Dawson and Foster (1982). The 

 more common species are discussed below. 



4.3.2 Species That Form Surface Canopies: 

 Kelp Forests 



4.3.2.1 Macrocystis . The morphology 

 and typical Laminariales life history of 

 Macrocystis is outlined in Chapter 1 (see 

 Figure 1). To grow into mature 

 sporophytes, microscopic spores must 

 alight on suitable substratum, develop 

 into gametophytes , become fertilized, then 

 grow from a microscopic sporophyte through 

 the water column to the surface of the 

 sea. The hazards encountered during the 

 course of this development are numerous 

 (see Chapter 5), and it is not surprising 

 that few, if any, of the billions of 

 spores produced by a single mature plant 

 ever make it through all of these stages. 

 Sedimentation, the pre-emption of space by 

 other species, the lack of light and 

 shading effects of other species, nutrient 

 limitation, and the effects of various 

 small and large grazers are some of the 

 factors which affect the growth and 

 survival of plants. 



Macrocystis and the other large 

 kelps, by virtue of their high growth 

 rates and sizes, are able to modify their 

 circumstances to a much greater degree 

 than smaller seaweeds. It is true to some 

 extent for any species that the 

 devastating effects of many factors may be 



outgrown. For example, a relatively small 

 amount of sediment on the substratum may 

 prevent the attachment of algal spores, 

 and may also remove over 98" of the 

 incident irradiance from reaching the 

 substratum (Devinny and Volse 1978, Norton 

 1978). Once a plant survives to a 

 juvenile stage of even a few millimeters 

 height, however, the effect of fine 

 sediment may be greatly reduced. The 

 larger members of the Laminariales and 

 Fucales can take advantage of the 

 progressively increasing light levels as 

 they grow through the water column. This 

 involves an increase in the spectra of 

 light available as well as the intensity 

 (Wheeler 1980a, Luning 1981). Blades that 

 lie near the surface of the sea may take 

 better advantage of sunlight, minimizing 

 the absorptive effects from sea water or 

 other species of algae. In the case of 

 Macrocystis , this is borne out by the fact 

 that most of the biomass is concentrated 

 at the surface in the upper 20%-30% of the 

 plant where most of the photosynthesis 

 occurs (Lobban 1978; Plates 1A, IB). 

 North (1972b) found that this concentra- 

 tion of biomass at the surface occurs 

 regardless of water depth. Translocation 

 through phloem-like sieve tubes moves 

 fixed carbon (primarily mannitol), and 

 proteins down surface fronds to the 

 holdfast and short, understory fronds 

 (Parker 1971, Lobban 1978). 



The early growth of Macrocystis 

 pyrifera in southern California has been 

 described by Neushul and Haxo (1963), 

 North (1971a), and Dean et al . (1983). 

 Giant kelp fronds may elongate at a rate 

 of over 30 cm/day, making it one of the 

 fastest-growing plants known (North 

 1971c). Neushul (1963) estimated that it 

 took 14 months from sporulation to 

 maturity for plants in southern 

 California. Plants may live up to 8 years 

 (North 1971a, Rosenthal et al . 1974). 

 Rosenthal et al. (1974) recorded the 

 survivorship of a cohort of Macrocystis 

 pyrifera plants in the Del Mar kelp 

 forest, over a period of 3.5 years (Figure 

 15). They found that there was a high 

 mortality rate in the first few months 

 after recruitment, but that this decreased 

 when plants were a year old. Five out of 

 the original cohort of 156 plants survived 

 after one year. Dayton et al . (1984) 

 constructed a life table for a Macrocystis 



46 



