(deeper) margin of the Point Loma kelp 

 forest has receded to depths shallower 

 than at the turn of the century, and 

 suggested that this may be due in part to 

 increased turbidity associated with the 

 nearby San Diego sewer outfall. 



The quality or spectral distribution 

 of light also changes with depth and water 

 characteristics (Jerlov 1968, Luning 

 1981); this may affect the distribution of 

 plants that require particular wavelengths 

 of light to maintain growth. In addition, 

 many kelps require a certain amount of 

 blue light for the induction of fertility 

 in gametophytes (Luning and Dring 1972, 

 Luning and Neushul 1978, Luning 1980). 

 For Macrocystis at 14 °C, a total of 

 between 1.5-12.3 E/m 2 of wavelengths 

 between 400 and 530 nM is required to 

 induce 50% fertility (Luning and Neushul 

 1978, Deysher and Dean in press). The 

 blue light response varies because it is 

 affected by the rate at which this light 

 is received; gametophytes have both 

 thresholds and saturation points (Deysher 

 and Dean in press). Work with other kelps 

 suggests that the amount of blue light 

 required also varies with temperature 

 (Luning 1980). Growth occurs without blue 

 light, but reproduction does not. 



Luning's (1981) suggestion that 1% of 

 surface irradiance is required for kelp 

 growth is an over-simplification because 

 light requirements differ for different 

 stages of the same plant, and these stages 

 occupy different depths and thus different 

 light regimes. The depth where 1% light 

 occurs can vary with water clarity and 

 canopy development. Table 2 lists light 

 requirements for growth in various stages 

 of Macrocystis . The values given should 

 be considered approximate as requirements 

 may vary with geographic location, temper- 

 ature, nutrients (Luning and Neushul 

 1978), water motion (Wheeler 1980b) and, 

 for photosynthetic measurements on adult 

 blades, position in the water column 

 (Wheeler 1980a). 



Growth will slow at light levels 

 below saturation, and fertility will 

 decline if less blue light is available. 

 Dean et al. (1983) estimated that 

 Macrocystis gametophytes outplanted on 

 artificial substrata in the San Onofre 

 kelp forest must receive the light 



Table 2. Downwelling light levels 

 (E/m 2 /day) for gametogenesis and compen- 

 sation (growth) and saturation (highest 

 growth rate) of growth in various stages 

 of Macrocystis . For reference, 1% of 

 surface light, suggested by Luning (1981) 

 as the lower light limit for kelp growth, 

 is about 0.2 E/m 2 /day. 



Stage 



Compensation Saturation 



Gametophytes 



Growth 3 , 

 Gametogenesis 



Young sporophytes c 

 (^ 1 cm long) 



0.3 2 



0.2 - 0.4 0.4 - 0. 



0.1 



1.5 



Juvenile sporophytes 

 (-X. 0.2-1 m long) 0.6 - 0.7 2-3 



Adults 



0.1 



10 



From Dean et al . 1983. 



From Deysher and Dean (in press). 

 .From Dean and Jacobsen (in press). 

 From photosynthetic rates in Clendenning 

 1971c. Light levels were converted from 

 foot-candles using conversion of Luning 

 1981, and assuming a 12-h day with con- 

 stant light. 



necessary to become fertile within about 

 40 days. Beyond this time, mortality due 

 to factors such as sedimentation and 

 grazing is apparently so high that few 

 gametophytes survive. This life history 

 stage can live and grow for much longer 

 periods in the laboratory (Sanbonsuga and 

 Neushul 1980), but whether gametophytes 

 can live longer on natural substrata in 

 the field is unknown. Larger stages may 

 survive longer at suboptimal light levels 

 because they are partly above the bottom 

 and not as affected by these factors. 



Adult Macrocystis plants are 

 generally insensitive to changes in 

 subsurface light because they usually form 

 a surface canopy, and can translocate the 

 products of photosynthesis toward the 

 holdfast (Parker 1963, Lobban 1978). 

 Younger stages are located on or near the 

 bottom, however, so a kelp forest could 



13 



