170 MARINE ANIMALS 



which is freed from the ammonium compounds by the considerable 

 "ammonia pressure," passing into the atmosphere and blowing over 

 the land, where it is absorbed by the soil. 



In any case, as for carbon dioxide, the nitrogen is available for 

 green plants only in the lighted upper levels. The nitrogen content 

 consequently also increases with depth; on the average, nitrogen as 

 nitrate and nitrite occurs in the oceanic surface water to the amount of 

 101 mg. per cu. m., 313 mg. at 400 m., and 485 mg. at 800 m. (Brandt) . 

 The loss of nitrogen in the upper layers is increased by the fact that 

 the bodies of many dead organisms sink far below the plant-inhabited 

 levels before they decompose. Thus in moderate depths, water near the 

 bottom of the sea is thought to be richer in nitrogen. Where such 

 bottom waters are brought to the surface, plant life will flourish. In 

 greater depths (5400 m.) Krogh found that the total nitrogen present 

 was approximately constant from surface to bottom at the time 

 tested. 11 There is conflicting evidence on this matter. 



The fertilization of the lighted upper levels of the ocean by nitrog- 

 enous compounds thus occurs in a variety of ways, principally through 

 the thorough mixing of the waters, the upwelling of deep water, and 

 the inflow of fresh water. The different amounts of these fertilizers in 

 different areas explains the difference in the abundance of marine life. 



A general mixture of marine water to the bottom occurs only in 

 shallow seas. It is produced in shallow coastal waters by the tides, 

 and to greater depths by storms. There is a periodical mixing of the 

 waters, in consequence of the cooling of the surface water in the winter 

 season, in the temperate and cold zones, which is sufficient to bring 

 up the nitrogenous water from the bottom in shallow seas. The mixing 

 extends to a depth of 140 m. in the Atlantic and to greater depths in 

 cold seas; to 800 or 900 m. south of Iceland. This is one of the reasons 

 why coastal waters in general are richer in animal life than the 

 near-by deeper seas with similar physical conditions (light and heat) , 

 and why shallow seas, like the North Sea and the western Baltic, and 

 shallow banks, such as the Doggerbank and Newfoundland banks, 

 have such a rich flora and in consequence a rich fauna, and thus be- 

 come favored fishing grounds. It also explains, in part, the greater con- 

 centration of organisms in the milder part of the Arctic and Antarctic 

 seas. Coloring of the sea by thick masses of plankton appears only in 

 shallow seas, as in the Baltic, and Gulf of Guinea, the Gulf of Cali- 

 fornia ("Purple Sea"), and the "red tide" of the Japanese coasts. The 

 deeper the ocean the more difficult such mixing of the sea water from 

 the surface to bottom becomes. 



On coasts where continued offshore winds carry away the surface 



