ZONES OF THE SEA-WHERE THEY LIVE 9 



as it commonly is on land. Rarely does the sea vary more than 70° Fahrenheit 

 at any one place. By contrast, land temperatures at Fairbanks, Alaska, vary from 

 60° below zero in winter to 105° in summer. 



If a di\'er were to progress downward through the water, even in warm seas, 

 he would reach, at a depth of 150 to 500 feet, a level where the water 

 suddenly became much colder. This place is called the "thermocline" and marks 

 the strong stratification between warm surface waters and cold deep waters. 



Temperature has several effects on water and on animals which are purely 

 physical. Cold waters are denser and have a greater viscosity than warm water. 

 Cold waters, therefore, sink below warm waters, and the temperature differences 

 become partly responsible for water movements in the ocean. Increased viscosity 

 of cold water means that planktonic animals sink more slowly in cold waters 

 than in warm. This may be part of the explanation for the huge concentrations 

 of plankton, particularly crustaceans, pteropods, and protozoans, in arctic or 

 subarctic waters or in deeper waters of warmer seas. These planktonic animals 

 are relatively scarce in shallow, warm waters. 



Features of the Sea 



It is easy to imagine the sea as a sort of featureless bowl, but since the 

 advent of sonic soundings about fifty years ago, it is now known that the 

 seas show all the features of the land— mountain ranges, canyons, valleys, vol- 

 canoes, etc. There are differences between the features of the sea and those 

 of land, however. The former are on a much grander scale but are barren and 

 comparati\'elv immutable. The gigantic deformities in the suboceanic crust of 

 the earth for the most part lurk in dark, silent, plantless depths, where erosion 

 is almost at a standstill. 



Geologically speaking, the ocean may be divided into three regions Cfig. i): 



1. Continental Shelf. This is a gradually sloping extension of the land under 

 water. It is formed by the deposition of river sediments as they are brought 

 from the land. As the ocean's water level rises and falls, the shelf is 

 alternatelv part of the underwater sea and part of the land. In some places, 

 such as the West Coast of North America, no continental shelf to speak 

 of is present, but the edges of most continents have shelves extending 

 outward from shore as little as. 10 or as much as 800 miles. The edge of 



the shelf is usually at a depth of 600 feet. The shelf fauna and flora is 

 the most varied in the sea. 



2. Continental Slopes. At the edge of the continental shelf the downward 

 pitch of the ocean bottom increases rapidly so that the water gets rapidly 

 deeper. These clifflike slopes and escarpments form intermediate depths 

 of the sea where protozoan oozes cover most of the bottom (Chapter 7). 

 The size of these escarpments so dwarfs anvthing of similar form on 

 earth that it is hard to comprehend them. They may descend from 600 

 to 18,000 feet, taking 100 miles of downward slope to do so. This repre- 

 sents a maximum size, but in some places the escarpments are even more 

 spectacular, being not as long but much steeper. 



