ZONES OF THE SEA-WHERE THEY LIVE 3 



many marine animals attain large size. Whales reach measurements that are 

 impossible on land; if they themselves are beached, they are unable to breathe 

 because of the great weight of their own bodies. 



Dissolved Salts 



For all of its remarkable qualities, pure water would not afford much of a 

 home for living things. The salts are the fertilizers of the seas, and without them 

 animals and plants could not grow. The seas of the world average about 35 

 parts per thousand of salt with extremes of 15 parts per thousand at the poles, 

 where melting ice dilutes the waters, and 46.5 parts per thousand in the Red 

 Sea, where there is a high evaporation rate and practically no dilution by rivers. 

 Most of the salts are sodium chloride (table salt), potassium chloride, and 

 magnesium chloride, but these are not the really vital ones. 



Nitrogen is vital in the building of protein. It is rather odd that animals 

 and plants are not able to utilize atmospheric nitrogen, even though nitrogen 

 composes about 80 per cent of air. That task falls to the incredibly numerous 

 nitrifying bacteria in the soil of land, which manufacture salts called "nitrites" 

 and "nitrates" from the nitrogen of air. These are then used by plants to build 

 protein. Eventually, however, many of these nitrites and nitrates find their 

 way into the sea by way of river systems. Allee, and others (1950) estimate 

 that the Mississippi River alone brings 792,000 tons of nitrate into the Gulf 

 of Mexico each year. This is the principal but not the only way that nitrogen 

 reaches the sea. Some nitrifying bacteria exist in shallow sea waters, but how 

 much they contribute is not known. 



Once in the sea, nitrites and nitrates are used by plants in the so-called 

 shallow-water "zone of nitrogen utilization." Animals eat the plants, and nitrogen 

 in the form of protein is distributed in the tissues of living things of the sea. 

 When these things die, they sink to the depths. Bacteria cause decay, and 

 nitrogen compounds are released in a deep-water "zone of nitrogen regeneration." 

 Nitrites and nitrates accumulate in the depths until they are swept to the surface 

 by currents in places where upwelling occurs. On the surface this rich fertilizer 

 once again becomes available to plant life. So we see that nitrogen in the sea, 

 is subject to an endless cycle, a cycle intimately connected to that of life and 

 death. 



The story of that other important fertilizer, phosphorus, is similar to that of 

 nitrogen. Once again, it is brought from the land to the sea by rivers, and again 

 it is used bv plants in a zone of phosphorus utilization. The plants are eaten by 

 animals. These living things sink to the deep sea when they die. Decay releases 

 phosphates in a zone of phosphorus regeneration and upwelling of deep waters 

 brings them to the surface. 



Immense as are the amounts of nitrogen and phosphorus compounds in the 

 sea, we must not think that they are inexhaustible. Just like any other fertilizers, 

 they can be used up. Every summer, as the sea warms and the plankton turns 

 the sea into a living soup, these fertilizers become progressively scarcer as the 

 abundant life uses them up. In some places the amount of such fertilizers limits 

 the amount of life that can grow in the water. When cold weather returns, 

 much of this life dies and the fertilizer returns to the depths. So the cycle of 



