4 UNDERWATER GUIDE TO MARINE LIFE 



nitrogen and phosphorus is one of the seasons as well as of life and death. 



Calcium is an important substance in the life of the sea but for a different 

 reason than was the case for nitrogen and phosphorus. It is also brought to the 

 sea by fresh water, existing in sea water as calcium sulfate largely, but it does 

 not build tissues. Instead, it builds skeletons. About 1.6 per cent of the salts of 

 the sea are calcium compounds. Many groups of animals, notably corals, sponges, 

 crustaceans, worms, molluscs, foraminifers, and some coralline plants are able 

 to precipitate soluble calcium compounds into relatively insoluble calcium 

 carbonate to build their skeletons. Since this reaction proceeds fastest at about 

 75° to 80° Fahrenheit, a common temperature range for tropical waters, it is 

 no surprise that coral reefs and the largest shelled animals are found in the 

 tropics. 



Silicon is similar to calcium. It is a skeleton builder, but it is used by only 

 a few groups, notably radiolarians, diatoms, and sponges. It exists in the sea 

 in the form of silicates. Silicates are the principal component of common glass, 

 and most siliceous skeletons have a decidedly glassy look. 



Nitrogen, phosphorus, calcium, and silicon are the four biologically most 

 important elements that are present in the sea in the form of dissolved salts. 

 There are other elements of lesser importance, such as iron and magnesium, 

 with similar case histories. 



Aside from their importance to life, salts have two important effects on 

 water. One is that they depress the freezing point of salt water from 32° 

 Fahrenheit to about 29°. The other is that salts cause water to be more dense. 

 Various salt concentrations are partly responsible for varying water densities. 



Every year about 660 million tons of earth are washed into the sea, and with 

 this dirt come great quantities of nitrogen, phosphorus, calcium, and silicon. 

 Therefore, it would seem that the sea is constantlv growing more and more 

 salty. Some scientists have assumed that the sea was originallv fresh water 

 and have even tried to compute the age of the sea by dividing the total tonnage 

 of salts in the sea by the yearlv deposition rate, but they have arrived bv this 

 method at very low figures for the age of the sea. The reason for this is that 

 deposition of these salts in the sea occurs at a varying rate, and salts are often 

 deposited permanently on the bottom. As salts are added by rivers, so they are 

 taken out of the water by various means. In fact, the sea has probably remained 

 about as it was with respect to salt content for the last billion years. 



There is some correlation between river deposits of salts and abundance of life. 

 The Atlantic and Arctic receive most of the North American river drainage and 

 have continental shelves to hold the deposits. Less river water flows into the 

 Pacific, it has no shelf to speak of. The comparative richness of life in the 

 Atlantic is due to this fact. The richness of some Pacific areas is due to another 

 factor, upwelling. 



Dissolved Gases 



Oxygen enters the sea primarily by being dissolved at the water's surface and 

 is distributed by water currents. Disturbed surfaces dissolve oxygen best simply 

 because more surface is exposed and because rough water acts as a good mixing 

 agent. 



