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The Great Chain of Life 



Where diatoms flourish, so do the fish. As 

 a basic food source for the p/ani<ton, 

 diatoms form rich pastures for fish. Those 

 shown on this page are magnified about 420 

 times. They grow attached to rocl<s and 

 seaweed and form chains. All diatoms live 

 in the upper layers of the sea and are kept 

 afloat by the shape of their delicate 

 sl<eletons, which are made of silica. 





Logically our discussion of life in the sea ought to have begun 

 with the plants. They form the pastures of green food on which all 

 animal life must depend, either directly or indirectly. But there are 

 two reasons why we did not begin with the plants. First, the sea- 

 weeds, which are a familiar and much-studied senior member of the 

 ocean's plant kingdom, are relatively unimportant to life in the sea. 

 The second reason is that little was known about the floating micro- 

 scopic green plants (that part of the plankton known as phyto- 

 plankton), which are the basic food source for the bulk of marine 

 animal life, until after the invention of the tow net. 



The enormous importance of the phytoplankton to the economy 

 of the sea cannot be too strongly stressed. The phytoplankton 

 forms the base of nearly all the food chains. It is vitally important 

 to all the harvests of the sea, whether these be the food fisheries, 

 sponge fisheries, whaling, or sealing. 



Plants alone can manufacture food from such inorganic chem- 

 icals as the minerals and gases. But they can do this only in the 

 presence of Ught, and light can penetrate water only to a Umited 

 extent. So for all practical purposes the vegetation of the sea does 

 not go below forty fathoms — which represents only a thin skin 

 over the oceans. Throughout that skin, in every part of the oceans, 

 float vast quantities of phytoplankton. The seaweeds, on the other 

 hand, are found only on a narrow fringe bordering the land masses. 

 One major exception, of course, is found in the floating seaweeds 

 of the Sargasso Sea; but in comparison with the phytoplankton, 

 the seaweeds are too sparse to have much effect on the economy of 

 the sea. 



This does not mean that man has made no use of them. Seaweeds 

 were once an important source of iodine. In many places where 

 the soil is barren seaweeds have been carted and spread over the 

 fields for use as fertilizer. Some have been used as food. For cen- 

 turies, too, the Japanese have been extracting agar-agar from sea- 

 weeds; the product is widely used in medicine, in bacteriological 

 work, for the making of jellies and emulsions. 



By the end of the eighteenth century many thousands of tons of 

 soda were being extracted from the brown seaweeds. With the 

 development of cheaper sources of soda in the early nineteenth 

 century, the industry died out; but after 1811, when iodine was 

 discovered, there was a small revival in the processing of kelps. 



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