disappear even if light was favorable for 

 adult growth. Light transmission to the 

 bottom is affected by the amount of 

 surface light, the water, dissolved and 

 suspended material in the water, and 

 shading by attached organisms. Surface 

 light intensity varies with latitude, 

 season, and cloud and fog cover, but the 

 range of intensities is much less than 

 that created by water characteristics 

 (Dean MS.). Day length may be important 

 in triggering photoperiodic reactions, but 

 this has not been investigated in subtidal 

 plants (Luning 1981). 



Light is attenuated logarithmically 

 with depth, and each wavelength has a 

 particular extinction coefficient (Jerlov 

 1968). The extinction coefficient also 

 varies with turbidity; in clear water, 

 blue light is transmitted further than 

 green light, while in more turbid coastal 

 water, the reverse occurs (Jerlov 1968). 

 Overall light transmission declines with 

 increasing turbidity and, within the 

 coastal water types designated by Jerlov 

 (1968), Luning (1981) estimated that the 

 depth where irradiance is reduced to 1% of 

 surface varies between 3 and 30 m. 



Water clarity or turbidity is 

 influenced by terrestrial runoff, 

 sediments resuspended by wave surge (Quast 

 1971c), plankton abundance (Quast 1971c, 

 Clendenning 1971b), and probably dissolved 

 and particulate matter produced by kelp 

 forest organisms (Clendenning 1971b). We 

 have observed the first three of these to 

 produce near darkness on the bottom in 

 kelp forests at mid-day. Moreover, 

 changes in water masses with changing 

 current conditions can cause rapid (less 

 than an hour) changes in water clarity. 

 For these reasons, short-term measurements 

 of light on the bottom, although useful in 

 comparing nearby areas at the same time, 

 should be used with caution in 

 characterizing the light regime of a site. 

 Light regimes, particularly if they are to 

 be used for correlations with algal 

 recruitment and growth, should be 

 determined with i_n situ continuous 

 recorders (Luning 1981; see Ramus in 

 press, and Foster et al. in press for 

 methods). 



Some of the earliest observations in 

 kelp beds and forests suggested that the 

 plants themselves have a great effect on 

 light reaching the bottom (e.g., Kitching 

 et al. 1934). Macrocystis canopies can 

 reduce irradiance by over 90% (Neushul 

 1971b, Dean et al. 1983, Reed and Foster 

 1984, Santelices and Ojeda 1984a), and 

 dense surface canopies of giant kelp are 

 often associated with a relatively sparse 

 understory algal flora (Dawson et al . 

 1960, Neushul 1965, Foster 1975b). Within 

 a locality, understory algal cover (Foster 

 1982a) and Macrocystis recruitment 

 (Rosenthal et al. 1974, Reed and Foster 

 1984) can vary inversely with Macrocystis 

 canopy cover. Pearse and Hines (1979) , 

 Reed and Foster (1984), and Dayton et al. 

 (1984), using experimental plant and 

 canopy removals, have demonstrated that 

 giant kelp canopies can inhibit the 

 recruitment and growth of the algae 

 beneath them. Moreover, natural kelp 

 recruitment usually coincides with times 

 when the surface canopy is reduced 

 (Rosenthal et al. 1974, Kimura and Foster 

 in press). Santelices and Ojeda (1984b) 

 also report an increase in Macrocystis 

 pyrifera recruitment when the surface 

 canopy was experimentally removed. In 

 contrast to some of the above studies, 

 however, understory kelp biomass 

 decreased. 



Understory kelps such as Pterygophora 

 cal ifornica , Eisenia arborea , and 

 Laminaria spp. cause further light 

 reductions, and the flora beneath stands 

 of _P. cal ifornica is often reduced to 

 articulated and encrusting corallines 

 (Reed and Foster 1984). At a constant 

 depth of 15 m, Reed and Foster (1984) 

 measured photon flux densities of 2%-6% of 

 surface in open water, . 2%-2. 5% under 

 canopies of either Macrocystis or 

 Pterygophora , and < 2% (usually <~ 0.5%) 

 under their combined canopies. sExperi- 

 mental removal of understory kelp canopies 

 can result in increased recruitment and 

 growth of plants, including Macrocystis 

 (Kastendiek 1982, Reed and Foster 1984, 

 Santelices and Ojeda 1984b, Dayton et al . 

 1984). Bottom-cover plants such as 

 articulated corallines also inhibit 

 recruitment, at least in part by further 

 reducing light (Reed and Foster 1984). 



14 



