NO. 9 MARINE INVERTEBRATES, ALASKA — MacGINITIE 23 



the same for any given latitude in all parts of the Arctic Ocean. Even 

 at the Pole, where the sun alternately shines for six months and is 

 below the horizon for six months, periods of twilight help to shorten 

 the time of total darkness to the extent that the growth of diatoms 

 can proceed throughout eight or nine months of the year. 



5. — The angle at which the sunlight enters the water ranges from 

 0° to 42° 10'. The greater the angle of the waves to the horizontal 

 the higher the angle at which light penetrates on the side toward the 

 sun. 



4. — The wind blows almost constantly at Point Barrow. Since 

 wind causes waves, the stronger the wind, the higher the waves, and 

 consequently the more the light penetration, to a limited extent. When 

 waves are sufficiently high to form whitecaps, light penetration no 

 longer increases with the strength of the wind. Prevailing northeast 

 winds set up wave surfaces toward the sun when it is at or near its 

 maximum height. 



5. — The weather is nearly always cloudy during the months of most 

 daylight. This low ceiling has considerable effect on the penetration 

 of light. The fog layer over the ocean is seldom over 20 or 30 feet 

 high. However, since the optimal condition for metabolism in diatoms 

 is subdued light, it is evident that the Arctic is an excellent place for 

 this group, at least during nine months of the year. Certain diatoms 

 were found all winter long under the ice but during midwinter even 

 those that were present were not abundant. 



6. — As has been said under "Ice," a considerable amount of the 

 ocean surface is frozen over. In addition to the shore ice, which ex- 

 tends 3 to 5 miles to sea, there is a great deal of floating ice, in which 

 leads come and go. Nevertheless, during winter the ice cover ranges 

 from almost 100 percent over a great portion of the Arctic Ocean to 

 at least 75 percent alongshore, that is, to a distance of 25 miles — the 

 limit of the present investigations. In summer, however, there is a 

 great deal of open water alongshore, and seldom, if ever, is more 

 than 25 percent of the surface covered by ice. The ice, like the 

 weather, is never the same from day to day or from season to season. 

 At least in the upper surface of the water the ice has much less effect 

 on light than might be supposed. In this investigation both ice houses 

 and tents were used in work on and through the ice. Within a tent 

 in January, even when the ice is 5 feet thick, a greenish glow comes 

 through it from below. The snow cover reflects light but at the same 

 time glows with transmitted light, and much light enters through it 

 into the ice and water below. 



7. — The effect of the penetration of light on plankton has been dis- 



