22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 128 



midnight it is still 4° 50' above the horizon. It is continually above 

 the horizon from May 17 to July 24 and continually below it from 

 November 19 to January 20. Because of the refraction of light at 

 the horizon, the sun can still be seen when it is actually below the 

 horizon. Twenty-four hours of daylight does not mean that there are 

 24 hours of sunlight. During much of the summer of 1948 there was 

 sunshine on the average of about one day a week. The sun may be 

 obscured by clouds all day and appear at 2 a.m. The amount of light 

 depends on the time of day the sun shines as well as on the length of 

 time it shines. If the sun is obscured during the time it is highest and 

 shines when it is near the horizon, there is less light than if it were 

 visible when at its highest and obscured when near the horizon. 



There are several factors that influence the amount of light in the 

 ocean waters off Point Barrow : 



1. The amoiont of sunlight. 



2. The amount of daylight. 



3. The angle with which light enters the water. 



4. The wind. 



5. The weather ceiling. 



6. The amount of ice. 



7. The amount of plankton. 



8. The amount of sediment in the water. 



I and 2. — Light may be one of the major factors affecting the life 

 of planktonic invertebrates in the Arctic Ocean. Though higher than 

 generally supposed, the metabolic rate of marine invertebrates is rela- 

 tively slow, thus enabling these animals to live longer under adverse 

 conditions than many other animals. Many of the more active ani- 

 mals, for example certain crustaceans and gastropods, store oil drop- 

 lets during the optimal conditions of late summer and early fall to 

 tide them over the winter. 



Light has little or no effect on the invertebrate bottom animals, be- 

 cause most of this fauna is below 100 feet. The detritus on which 

 these animals feed is not greatly affected, if at all, by light, bacterial 

 action being more dependent upon temperature. As temperature is 

 so uniform in the Arctic, these animals have a constant supply of food 

 independent of light, and are therefore adapted to continuous dark- 

 ness. The planktonic animals are adjusted to a long period of semi- 

 darkness. 



In general the light entering the waters of Point Barrow is by no 

 means so intense as that in more southern regions, but, being present 

 over longer periods, it is sufficient for the photosynthesis of diatoms, 

 which proceeds at an optimal rate in subdued light. Light is exactly 



