have different physiological tolerances to 

 regimes of light, temperature and exposure 

 (e.g., Quadir et al . 1979). Even so, 

 individual algal species may not occupy 

 the zone where they grow best (e.g., 

 Foster 1982b). Detailed experimental 

 studies examining these types of 

 interactions have rarely been done in 

 subtidal algal forests. 



Within sites, it has been observed 

 worldwide that subtidal algal assemblages 

 fall into broad depth zones (Kain 1963, 

 Neushul 1965, Mann 1972a, Choat and Schiel 

 1982), and this is generally the case for 

 Macrocystis communities (see Chapter 3). 

 Most of the physical and biotic factors 

 discussed in Chapters 2 and 3 will change 

 gradually along a depth gradient within 

 any one site. For example, the density of 

 kelp stands usually decreases at depths 

 beyond ^ 20 m, leaving isolated plants at 

 greater depths (DeVinny and Kirkwood 1974; 

 see Chapter 1). The pattern of distri- 

 bution of physical factors can also be 

 quite variable. While light attenuation 

 occurs with depth, it may also be affected 

 by water clarity and the presence of algal 

 canopies (see Section 2.4). As a 

 consequence of changing variables and the 

 physiological tolerances of algae, the 

 demography of species and composition of 

 algal stands also change with depth. 



The effects of depth-associated 

 factors on algal production have been 

 approached in several ways. North (1971c) 

 measured the growth rates of three young 

 Macrocystis fronds on each of four plants 

 over a period of 39 days at depths of 8 m 

 and 24 m on the edge of the Carmel 

 submarine canyon. He found little 

 difference among elongation rates of 

 fronds, and concluded that the influence 

 of depth on growth rates of young fronds 

 is probably slight. Sample sizes were 

 small in this study, and only young fronds 

 on older plants were measured. Gerard 

 (1976) found that within one site in 

 central California, the variation in 

 growth rates was extremely high from frond 

 to frond, even for fronds which were 

 initially of equal size and which were 

 measured over the same time interval. 



Dean et al . (1983) at the Marine 

 Review Committee kelp ecology project in 

 southern California have done the most 



innovative and thorough research to date 

 examining factors that affect the 

 production of Macrocystis sporophytes from 

 gametophytes. Their experiments will be 

 discussed in detail in the next section. 

 Of relevance to the depth distribution and 

 abundance of Macrocystis , however, were 

 their experiments that assessed the 

 production of sporophytes at different 

 depths. They attached ropes containing 

 cultured gametophytes at different depths 

 along a suspended cable (Figure 28). The 



bottom 

 plates 



a = Floating PVC frame 



b = Plexiglass plates holding pieces of 



nylon rope containing gametophytes 

 c = Surgical tubing with stainless steel 



hooks used to attach plates to frame 

 d = Sediment tube 

 e = Irradiance sensor 

 f = Irradiance-temperature integrator and 



logger 

 g = Stainless steel and safety cable 

 h = Iron anchor plate 



Figure 28. Field station used for Macro - 

 cystis pyrifera gametophyte outplants 

 (Dean et al. 1983). 



94 



