ZONES OF THE SEA-WHERE THEY LIVE 5 



Plants also furnish oxygen to the sea by the process of photosynthesis. This 

 reaction requires light, however, so it occurs only in approximately the first 

 250 feet of water. As the light diminishes with depth, plants become oxygen 

 users, not producers. At about 100 feet, plants use about as much oxygen as 

 they release, and below that depth the few plants that exist become dependent 

 upon oxygen, releasing very little. The 100-foot depth is called the "compensa- 

 tion level," and it xaries with light conditions, latitude, time of day, and water 

 claa'ity. 



Oxygen is extracted from water by the respiration of all forms of life and 

 bv oxidation and decay of dead bodies through the agency of bacteria. Allee, 

 and others (1950) state that bacteria probably use more of the sea's oxygen 

 than all other forms of life combined. 



The concentration of oxygen in the sea is highest at the surface, where 

 solution and photosynthesis take place and where the water may even become 

 supersaturated. This is particularly true of brightly lit surface waters in mid- 

 afternoon when photosynthesis is at its peak. At a depth of about 1,500 feet, 

 oxygen falls to its lowest level because it is not produced there, yet enough life 

 is present to use what there is rapidly. Oxygen increases below the 1,500-foot 

 level because it is carried downward by sinking cold vvaters from polar seas. 



In the comparatively few deep spots where overturn of water cannot occur 

 and where sinking cold water cannot reach bottom, the water may become 

 stagnant. All of its oxygen is used, and the poisonous gas hydrogen sulfide 

 accumulates from the decay of dead bodies that sink there. The Black Sea is 

 the most famous body of water that accumulates hydrogen sulfide. Practically 

 no life at all occurs there from a depth of 600 to 6,000 feet. The Bay of Naples 

 and several deep fiords of northwestern Europe also tend toward stagnancy. 



Carbon dioxide is the third major gas in the sea. It is a fertilizer similar to 

 the salts of nitrogen and phosphorus. It is released into the sea by the respiration 

 of all animals and by plants deprived of light. It is dissolved by surface waters in 

 the same way that oxygen is dissolved from the atmosphere. It is also released 

 at the bottom of the sea by decomposition of dead bodies and, like nitrogen 

 and phosphorus compounds, accumulates in deep waters. Upwelling brings 

 this carbon dioxide to the surface. 



Carbon dioxide is used in two very important ways. First, it is used by plants 

 in photosynthesis to build carbohydrates (sugars and starches). Second, it is 

 used in the converted form of carbonates and in combination with calcium to 

 build the limy skeletons of many invertebrates. 



Pressure and Depth 



Next to the yvetness of water itself, the most noticeable effect of water on 

 the diver is that of increased pressure as he descends. The pressure of air at 

 sea level is one atmosphere or about 15 pounds per square inch. For every 33 

 feet of descent into the sea, the pressure increases one atmosphere. At the 

 deepest part of the ocean, animals are subject to about 1,000 atmospheres of 

 pressure or 15,000 pounds per square inch. This immense pressure would be 

 quite disastrous if applied unevenly to a body, but it has no effect when it is 

 exerted in all directions inside the body and out. To illustrate: each square inch 



