CHAPTER 3 

 THE GIANT KELP COMMUNITY 



The biotic components and temporal population changes 

 recorded off Del Mar should not be interpreted as "charateristic" 

 of all southern California kelp beds. Rosenthal et al. (1974). 



3.1 INTRODUCTION 



We have defined giant kelp forests as 

 subtidal communities composed of Macro- 

 cystis pyrifera and associated organisms. 

 Although by definition, Macrocystis is 

 always present in these communities, its 

 local distribution and abundance vary in 

 time and space, as do the distribution and 

 abundance of other plants and animals 

 associated with it (Clarke and Neushul 

 1967, Rosenthal et al. 1974, Foster 

 1982a). In addition, within the 

 geographic range of Macrocystis forests, 

 the species composition of associated 

 organisms can also vary; their geographic 

 ranges are not necessarily the same as for 

 Macrocystis . So far as is known, no 

 organisms found in stands of giant kelp 

 have an obligate association with 

 Macrocystis ; they can be found in other 

 kelp communities and on subtidal rocky 

 reefs devoid of large brown algae 

 (Pequegnat 1964, see Chapter 4). In this 

 chapter we describe the community 

 structure of a number of sites, discuss 

 the spatial and temporal variability of 

 this structure, and review kelp forest 

 energetics. 



To aid the discussion that follows, 

 Figure 6 illustrates a "composite" giant 

 kelp community with emphasis on the large, 

 visually obvious kelps that provide much 

 of the structure of the community, and for 

 which we have the most descriptive 

 information. The figure divides the 

 community into habitats within giant kelp 



forests where particular organisms typi- 

 cally co-occur. It is a composite, both 

 because of local variation in species 

 distribution, and because the geographic 

 ranges of some of the organisms, such as 

 sea otters ( Enhydra lutris ) and the elk 

 kelp ( Pelagophycus porra ) , do not 

 presently overlap. The figure does, how- 

 ever, indicate the potential complexity of 

 the community, with multiple layers of 

 vegetation (Dawson et al. 1960, Foster 

 1975a), over 50 species of fishes that 

 commonly segregate into various 

 microhabitats (Quast 1971a, Miller and 

 Geibel 1973, Feder et al. 1974, Ebeling et 

 al. 1980a), and numerous invertebrates 

 also found in particular habitats (e.g., 

 on plants, on vertical or horizontal 

 surfaces, and on holdfasts). North 

 (1971b) listed 130 species of plants and 

 almost 800 species of animals associated 

 with giant kelp in southern California and 

 northern Baja California, Mexico. The 

 giant kelp holdfast alone may contain over 

 150 species (Ghelardi 1971). A variety of 

 birds and mammals forage in the community, 

 including cormorants, harbor seals, and 

 sea otters. Kelp forests also contain a 

 planktonic assemblage of generally 

 microscopic organisms, many of which are 

 stages in the life histories of larger 

 members of the community. 



As indicated in Figures 3, 6, 9, and 

 10, Macrocystis communities generally 

 occur within a narrow depth range. Even 

 if suitable substrata are available, M. 

 pyrifera usually does not occur shallower 



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