Chapter II 



THE MARINE ENVIRONMENT 



The distribution, characteristics, and activities of microorganisms in 

 the sea are influenced by the marine environment. In turn, certain en- 

 vironmental conditions in the sea are influenced by the activities of micro- 

 organisms. Therefore a knowledge of the essential features of the environ- 

 ment is prerequisite to the study of marine microbiology. 



Extent and depth of the sea: — The oceans and seas cover 143,000,000 

 square miles, or about 71 per cent of the earth's surface. The average 

 depth of this water is nearly 12,500 feet as compared with a mean eleva- 

 tion of the land above sea level of 2,300 feet. Whereas the greatest land 

 elevation, Mount Everest in the Himalayas, is 29,000 feet above sea level, 

 the greatest known depression on the earth's surface, the Mindanao Deep 

 off the Philippine Islands, is 35,400 feet below sea level. 



According to data compiled from Kossinna by Sverdrup et al. (1942), 

 92.4 per cent of the ocean area, including adjacent seas, exceeds 656 feet 

 or 200 meters in depth. The percentage area of depth zones is recorded 

 in Table I. 



Table I. — Percentage area of depth zones in the oceans, including adjacent seas, from 

 Sverdrup et al. (1942): — 



Topography of the sea floor : — The topography or profile of the ocean 

 bottom is very irregular. There are submerged mountain-like structures 

 of various dimensions which are termed ridges or rises, some of which 

 rise precipitously for several thousand feet. Some, whose tops extend 

 above the water, form islands. The term sill is applied to a submerged 

 elevation separating two basins. Ridges, rises, and sills influence the 

 movement of water masses and thereby exert a pronounced effect upon 

 the physical, chemical, and biological properties of the water. Certain 

 elevations restrict the migrations of marine animals. For these and re- 

 lated reasons the topography of the sea floor must be taken into account 

 in the study of the ecology and activities of marine organisms. 



Large scale depressions are called troughs, trenches, or basins. A deep 

 is the lowest part of a depression, being the antithesis of a mountain peak. 

 Canyons are relatively long narrow depressions or furrows in the conti- 

 nental shelf. Submarine canyons range in size from small gullies to vast 

 depressions equalling the Grand Canyon of the Colorado River in all 



