172 MARINE ANIMALS 



and Ionian seas (cf . p. 154) . Downward currents attract surface water 

 whose nitrogen and carbon dioxide have been exhausted, and areas 

 where they occur, like the Sargasso Sea, are poor in life. The small 

 importance of thermal currents in the Mediterranean, whose waters 

 are no colder than 12.9° at the bottom, results in general poverty in 

 plankton despite local exceptions. Many of the relations of phosphorus 

 and silicon are similar to those which have been discussed for nitrogen. 

 In the higher latitudes distinct seasonal fluctuations of the amounts of 

 these essential mineral nutrients occur which are accompanied by sea- 

 sonal variations in the quantity of plankton organisms. 



Influence of rivers. — The distribution of rich faunae in the ocean 

 is influenced also by the amount of materials received from the land, 

 principally from the rivers. This is probably an additional factor in 

 the wealth of marine life in coastal waters, in addition to those before 

 mentioned. Such materials are very unequally distributed in the oceans. 

 The Atlantic and Arctic receive the largest amounts of river water. 

 They receive the waters of all the European rivers and of the most 

 important African rivers; almost the whole of both American conti- 

 nents drains into the Atlantic; and the waters of the mighty Siberian 

 rivers, draining into the Arctic, are carried toward the coast of Green- 

 land by a westward current. The Atlantic-Arctic is thus surrounded 

 by tributary land masses, and with a surface of 103,000,000 sq. km., 

 receives the drainage from over half of the land surface of the earth. 

 The Atlantic has in addition wide shallow areas, especially at the 

 north, with 26% of its extent within the 100-fathom line and only 

 40% deeper than 1000 fathoms and so is well equipped to produce 

 maximum returns from its enriched waters. The Indo-Pacific-Antarctic, 

 on the contrary, receives its land waters almost entirely from Asia; 

 with 265,000,000 sq. km., it receives the drainage of only 27% of the 

 land surface. The southeastern Pacific is least supplied with land 

 waters, and this source of fertility is almost negligible. The result is 

 that the Atlantic differs from the Pacific Ocean biologically as well as 

 geographically. 



These facts help explain the poverty in life of the open southeastern 

 Pacific, though it must be added that almost three-fourths of the extent 

 of the entire Pacific Ocean is more than 3600 m. deep and that the 

 absence of a continental shelf on the west coast of the Americas, with 

 the lack of coastal habitat differentiation, contributes to the same 

 result. The waters of the Pacific have the lowest content in nitrogen 

 compounds. The surface water contains, per cubic meter (the figures 

 in parentheses refer to the tropical belt between 24°S. and 25°N.) : 



