California, Mexico, to Newport Bay in 

 southern California survived better during 

 periods of warm water than native plants. 

 Deysher and Dean (pers. comm. ) are 

 evaluating the growth and fertility of 

 Macrocystis pyrifera gametophytes produced 

 from adults collected in Baja California, 

 Mexico (warm water), the San Diego area 

 (moderate-warm water), and Monterey (cold 

 water). In contrast to the findings of 

 North (1972b), their preliminary results 

 indicate all gametophytes behave similar- 

 ly, and sporophyte production occurs at 20 

 °C if nutrients are adequate. Growth and 

 fertility decline rapidly above 23 °C in 

 all plants. More of these kinds of 

 experiments, combined with adequate 

 temperature records from geographic bound- 

 aries, are needed to evaluate 

 temperature-distribution hypotheses 

 critical ly. 



Temperature has also been suggested 

 as being important to the geographic 

 distribution of kelp forest fishes and 

 invertebrates (Quast 1971a, Gerrodette 

 1979). As for bull kelp and gorgonian 

 corals (see above), major changes in the 

 distribution of kelp forest species occur 

 near Point Conception where the California 

 Current moves offshore, creating large 

 changes in temperature within a short 

 distance. Briggs (1974) also emphasized 

 the importance of these changes near Point 

 Conception in his review of marine bio- 

 geography. However, most of the 

 relationships between distribution and 

 temperature are based on correlative 

 evidence, and the difficulties mentioned 

 above for algal distribution also apply to 

 these other kelp forest organisms (see 

 Gerrodette 1979). 



2.4. LIGHT 



The methods of measuring light 

 relative to the biology of kelp forest 

 organisms have undergone numerous changes 

 in recent years, and an understanding of 

 these changes is necessary to interpret 

 the results of light studies. Luning 

 (1981) recently reviewed this subject, so 

 we will only briefly summarize it here to 

 aid the discussion that follows. 



Marine plants have a diverse array of 

 light-absorbing pigments such that wave- 



lengths of between roughly 400 and 700 nM 

 are used in photosynthesis (so-called PAR 

 or Photosynthetical ly Active Radiation); 

 other plant processes may be sensitive to 

 light outside this range (Luning 1981). 

 Inexpensive, portable instruments to 

 measure light in this region of the 

 spectrum were not available until 

 recently, and most early measurements were 

 made with photometers that measure light 

 (illuminance) in foot-candles (English), 

 or lux (metric; 1 f-c = 10.764 lux). 

 Illuminance is based on the sensitivity of 

 the human eye, and measurement instruments 

 are designed with maximum sensitivity in 

 the green region of the spectrum (550 nM). 

 Therefore, photometers do not properly 

 measure the light actually available for 

 photosynthesis. Later measurements have 

 been made with instruments that detect all 

 portions of the spectrum with equal 

 sensitivity in energy units (such as 

 watts/m 2 ; irradiance). With proper 

 filters, these instruments can measure 

 just PAR. Light quantity can also be 

 measured as photon flux density. This is 

 a particularly appropriate unit because 

 photosynthesis is a quantum process. 

 Instruments are now available which 

 measure photon flux density of PAR. The 

 units are Einsteins/area/time or 

 mols/area/time, where 1 Einstein = 1 mol = 

 6.02 x 10 23 photons. Unless otherwise 

 noted, light measurements below are photon 

 flux density of PAR. 



Adequate light is essential for the 

 growth of Macrocystis and other plants 

 within a kelp forest, and may affect the 

 behavior of other organisms such as fishes 

 (Quast 1971c). Most seaweeds in a kelp 

 forest at least start life on the bottom, 

 and light, as well as other factors 

 affecting plant growth, must be suitable 

 there. In the absence of other possible 

 controlling factors (presence of sand, 

 grazers, etc.), the lower depth limit of 

 giant kelp and, therefore, giant kelp 

 forests, is probably determined by light. 

 Luning (1981) suggested that for most 

 kelps, this limit will occur where 

 irradiance is reduced to ^ 1% of that at 

 the water's surface. Giant kelp generally 

 grows deeper in clearer water as seen in 

 some central California kelp forests where 

 depth distributions are correlated with 

 water clarity (Foster 1982a). Barilotti 

 (pers. comm.) notes that the outer 



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