14 UNDERWATER GUIDE TO MARINE LIFE 



and the other rises). But these causes are probably minor in comparison to the 

 effect of winds. 



The net result of all these massive water movements is the production of 

 rather stable, but exceedingly complex water masses in the seas, which may be 

 identified by characteristics of temperature and salinity. The complexities of 

 their movement are introduced largely by the lay of the land, that is, con- 

 formation of the ocean bottom and of the land masses. For instance, the best 

 known of the horizontally moving water masses, the Gulf Stream, is a distinct, 

 blue river of water, 1,200 times the size of the Mississippi, which pours out 

 of the warm Gulf of Mexico through the Florida Strait at a speed of up to 

 6 mph. It proceeds north to Cape Hatteras, where it is deflected out to sea, 

 runs into the cold Labrador Current near Nova Scotia, and breaks up into four 

 separate currents before reaching England. Water returns to the Gulf of Mexico 

 by the Caribbean Current to the south. Similarlv, the movements of vertical 

 water masses are influenced by the conformation of the bottom. Most of the 

 cold bottom water of the mid-Atlantic is supplied from the antarctic because 

 those of the arctic are blocked by underwater barriers. Local rain, river influx 

 into the sea, melting ice, and sun also have effects on densitv which greatly 

 complicate the picture. 



In summary, the movements of water have, no doubt, a greater effect on 

 the abundance and distribution of marine life than any other physical factor. 

 The oscillating surface factors of waves and tides are mainly responsible for 

 mixing oxygen and carbon dioxide at the surface. In shallow water, they buffet 

 the living and nonliving alike, either building or eroding shore lines and forcing 

 the sessile, or sedentary, animals and plants that live under the impact of their 

 blows to become either massive and thick like corals and sea shells, or flexible 

 and resilient like seaweeds and gorgonians. The nonoscillating water movements 

 are composed of an interplay between two rotating systems, one which is 

 horizontal and caused by winds and one which is vertical and caused by winds 

 and varying water densities. These produce circulating water masses and serve 

 to mix dissolved salts and gases and to distribute animals, particularly planktonic 

 ones, throughout the seas of the world. 



ZOOGEOGRAPHY: Communities of Marine Life 



It is almost too obvious to point out that the sea is not evervvvhere the same, 

 but simple statements are often the most important ones. The studv of the 

 various environmental differences and similarities that exist in the sea or on 

 the land and the precise recognition of their characteristics allows the recognition 

 of definite zones, that is, fairly sharplv delimited areas where a certain set of 

 physical features prevail— certain temperature limits, water movements, depths, 

 light, etc. One such type of zone is the tidal, characterized by movements of 

 waves and tides. Another type is the eu photic or lighted zone, discussed in the 

 section on light. Another is the temperate, delimited bv certain temperature 

 extremes. 



Any zone will have living in it a group of animals and plants that have 

 become specialized to meet the physical and biological requirements of its zone 

 in order to live more efficiently there. All of these forms of life are grouped 



