394 THE ALGAE 



It seems evident that algae of the arctic (and antarctic) can toler- 

 ate as much as 80 per cent of the internal water being frozen (with 

 the consequent rise of internal concentration) without metabolic 

 processes stopping. After many months frozen into sea ice they 

 must be capable of photosynthesis immediately upon being thawed 

 out. 



(d) Illumination 



Here one may be faced with problems involving seasonal varia- 

 tion in Ught intensity, diurnal variations in relation to times of high 

 water, and the actual light intensity and spectrum at different 

 depths. Practically nothing is known about the first two effects, and 

 it is impossible therefore to assess their importance. It can, how- 

 ever, be pointed out that differences in illumination intensity at 

 different latitudes appear to determine the maximtim depth to 

 which algae can descend. Thus in the North Sea region the limit 

 appears to be about 40-50 m., in the tropics it is about 100 m., 

 whilst in the clear waters of the Mediterranean it is about 130- 

 180 m. Many of the deep-growing algae (and possibly others) are 

 fully hght saturated at quite low values. Thus blades of Macro- 

 cystis are fully saturated at 1000 foot-candles (ro full sunlight of 

 CaHfornia) and parts of Egregia at 1400 foot-candles. 



The third effect is undoubtedly important because measurements 

 show that the incident Hght is cut down very considerably at even 

 a depth of i m. whilst at 2 m. only about 25 per cent of the surface 

 Ught has penetrated. Waters of different turbidity may absorb light 

 quite diflferendy (Fig. 212A) and this in turn affects the relative 

 photosynthetic rate and the compensation point level (Fig. 212B). 

 During its passage through sea water the spectral composition of 

 the Hght changes considerably, the green and blue portions as- 

 suming greater importance. Algae Hving in the sub-Httoral fringe 

 and the upper portion of the subHttoral are subject to very different 

 conditions as compared to algae Hving higher on the shore, so far 

 as photosynthesis is concerned. There is also the different effect of 

 submergence and emergence upon photosynthesis, the process 

 being affected not only by the Hght change but also by the moisture 

 and temperature changes. 



In any given locaHty it is evident that temporary changes in Hght 

 intensity as a result of cloudiness or chop on the water may affect 

 photosynthesis. The outer edges of Macrocystis beds in California 



