Kelps that produce surface canopies 

 usually have floats (pneumatocysts) or 

 other gas-filled structures for buoyancy. 

 Numerous other kelps lack such structures 

 and do not form a surface canopy except at 

 low tide if they are growing in very 

 shallow (1-2 m deep) water. They may, 

 however, form dense canopies up to a few 

 meters above the substratum to which they 

 are attached. Extensive stands of these 

 smaller laminariales occur in all 

 temperate and some polar regions of the 

 world. Conventionally, these stands are 

 referred to as kelp "beds," recognizing 

 subjectively that the vertical structure 

 is not so extensive as it is for those 

 species forming surface canopies. Com- 

 munities featuring dense stands of 

 Laminaria spp. have been studied in Nova 

 Scotia ("Mann 1972a, b, Breen and Mann 

 1976, Chapman 1981), and the British Isles 

 (Kain 1979). In the southern hemisphere, 

 Ecklonia spp. have been studied in New 

 Zealand (Choat and Schiel 1982), Australia 

 (Shepherd and Womersley 1970, 1971, 

 Kennelly 1983), and South Africa 

 (Velimirov et al. 1977). These beds are 

 analogous to kelp forests but without a 

 surface canopy, and the principles of 

 sampling and experimenting in them should 

 be no different than in giant kelp for- 

 ests. We have drawn on this literature 

 for comparisons with work in Macrocystis 

 communities. 



Macrocystis belongs to the Order 

 Laminariales, Family Lessoniaceae. It has 

 a typical laminarian life cycle (Figure 

 IB), with microscopic haploid spores (IN) 

 developing into male and female gameto- 

 phytes (IN). Eggs on female gametophytes 

 are fertilized, and the zygotes develop 

 into the macroscopic diploid sporophytes 

 (2N). On mature plants, clusters of 

 sporophylls containing reproductive tissue 

 (sori) are found on the lower portion of 

 plants just above the holdfasts (Figure 

 lb). Fronds (stipes with associated 

 floats and blades) comprise the large 

 portion of plants which are seen in the 

 water column, and floating on the surface 

 of the sea. Further descriptions of the 

 life cycle and morphology of Macrocystis 

 can be found in Abbott and Hollenberg 

 (1976) and Lobban (1978). 



1.2. DISTRIBUTION 



Subtidal forests of Macrocystis occur 

 in many areas of the world, but are most 

 widely distributed in the southern hemi- 

 sphere (Figure 2). Populations exist in 

 the southern hemisphere along the east and 

 west coast of South America, off South 

 Africa, Tasmania and south Australia, New 

 Zealand, and the sub-antarctic islands. 

 As with other species with bipolar distri- 

 butions, it is generally thought that 

 temperature is the chief barrier to the 

 geographic expansion into warmer waters 

 (Hedgpeth 1957). However, recent work has 

 shown an inverse relationship between 

 temperature and nutrients, so low nutri- 

 ents may be the important factor (see 

 Sections 2.3 and 2.5). In contrast, 

 Gaines and Lubchenco (1982) suggest that 

 herbivory increases inversely with lati- 

 tude, so extension into warmer waters may 

 also be limited by grazing. A combination 

 of cold water and low light levels acting 

 on the various life history stages of 

 plants probably prevents expansion of 

 ranges toward the poles (Van den Hoek 

 1982). North (1982) suggested that the 

 genus evolved in the southern hemisphere 

 and its bipolar distribution in the 

 Pacific (but not the Atlantic) may be the 

 result of a cold water "bridge" in the 

 geologic past (North 1971b). In contrast, 

 Estes and Steinberg (MS.) point out that 

 all but one of the presently recognized 

 genera in the Order Laminariales occur in 

 the North Pacific, and suggest a North 

 Pacific origin and subsequent southern 

 migration for these kelps, including 

 Macrocystis . 



Populations of Macrocystis in the 

 North Pacific extend from Alaska to local- 

 ities of upwelled cooler water in Baja 

 California, Mexico (Druehl 1970; see 

 Chapter 3). Macrocystis pyrifera is found 

 from near Santa Cruz in central California 

 to Baja California, Mexico (Druehl 1970). 

 Abbott and Hollenberg (1976) give Alaska 

 as the northern limit, but Druehl (1970) 

 is probably correct as we have never 

 observed the species much beyond Santa 

 Cruz, and could find no other reports of a 

 more northern distribution. 



