the surface during summer and is destroyed in 

 autumn and winter when vertical mixing creates a 

 layer of relatively uniform temperature in the upper 

 20 to 300 meters. 



Currents moving towards the poles from the 

 equator consist of warm water, and currents moving 

 in the opposite direction of cold water. Cold waters 

 flowing towards the equator tend to be deflected to 

 the right and hence bathe the North Atlantic coast 

 of North America and the North Pacific coast of 

 Asia. In the southern hemisphere they come into 

 contact with the west coasts of both South America, 

 Africa, and Australia. Warm waters, on the other 

 hand, bathe the west coasts of Europe and North 

 America and the east coasts of Australia, Africa, 

 and South America. 



Organisms living in the intertidal zone on the 

 shore are ordinarily exposed to great variations of 

 temperature twice during each day as they are alter- 

 nately flooded by the tides and exposed to the air and 

 direct solar radiation. Unusually severe cold spells 

 during the winter have been known to produce exten- 

 sive mortality of fish and invertebrates in shallow- 

 waters of? the coasts of Texas and Florida (Gunter 

 1941). On the other hand, one of the characteristic 

 features of the deep-sea habitat is its low and almost 

 constant temperature. 



Light 



The character of the radiation, as well as its 

 intensity, varies with depth. Even in the clearest 

 waters and at maximum radiation, the red, orange, 

 and ultraviolet are absorbed in the first 20 m. Green, 

 yellow, and blue wavelengths penetrate farther, de- 

 pending on the water color. When the sun is not at 

 the zenith, light penetration is reduced, and the maxi- 

 mum penetration in the winter at high latitudes is 

 much less than during the summer (Clarke 1939, 

 Jerlov 1951). 



The compensation point, or the depth at which 

 the amount of oxygen released in photosynthesis by 

 algae just balances the oxygen needs of the plants 

 for respiration over 24 hours, has been found to vary 

 during the daytime between 1 and 100 m, according 

 to the locality, turbidity, and season. The upper illu- 

 minated layer where photosynthesis exceeds respira- 

 tion is often called the plwtosynthetic zone (Harvey 

 1955). 



Salinity 



The salinity of sea water varies from place to 

 place depending largely on the amount that it is di- 

 luted by the inflow of fresh water from rivers or 



melting glaciers or the amount that it is concen- 

 trated by evaporation. The Red Sea, for instance, 

 has a salinity of 40°/00 (40 g dry salts in 1000 g sea 

 water) while in some polar seas the salinity is less 

 than 30°/00. The average salinity of the oceans as 

 a whole is commonly given as 35°/00 of which the 

 chloride ion constitutes about 19°/00 and the sodium 

 ion a little over 10°/00. The various major salts 

 occur nearly everywhere in definite and constant pro- 

 portions. As one would expect, the pH of sea water 

 is high, averaging about 8. There is some similarity 

 in relative proportions and concentrations of the vari- 

 ous ions in sea water and in the blood or body fluids 

 of many invertebrate organisms. This may indicate 

 that the sea is the habitat in which living forms first 

 evolved. 



The contrast in salinity between sea water 

 (35,000 ppm) and fresh water (15-660 ppm) re- 

 quires important differences in physiological adjust- 

 ment of organisms to occupy these two habitats. The 

 problem is one of osmotic regulation (Black 1951). 

 Most marine invertebrates are poikilosmotic in that 

 they are nearly isotonic with sea water, they are 

 highly permeable to water, and gain or lose water 

 according to the concentration of the medium. A few 

 marine segmented worms, flatworms, and crabs and 

 all marine fish and mammals have at least some in- 

 ternal osmotic regulation and tend to be homoios- 

 motic. All except the elasmobranch fishes maintain 

 body fluids hypotonic to sea water in various ways. 

 The skin has decreased its permeability to the free 

 movement of water back and forth, the necessary 

 water is obtained by swallowing, surplus salts are 

 secreted outside the body, especially through the 

 gills, and there is a general decreased function or 

 atrophy of water secreting organs such as the kid- 

 neys. The practical absence of insects and amphibians 

 from the sea is largely due to their inability to secrete 

 salts outwardly. The high osmotic concentration 

 found in elasmobranchs is the result of huge quan- 

 tities of urea retained in the body tissues and fluids. 



Fresh-water organisms, in contrast to marine 

 forms, maintain body fluids hypertonic to the sur- 

 rounding medium by excretion of water through con- 

 tractile vacuoles in lower organisms or highly func- 

 tioning kidneys in higher ones, active absorption of 

 salts from the surrounding water by special cells in 

 the gills, and reabsorption of salts from the urine. 

 There is no swallowing of water, as sufficient amounts 

 are absorbed by osmosis through the gills and mouth 

 surfaces and incidentally with feeding. 



Probably the most extensively utilized of the dis- 

 solved substances in the sea are the nitrogen com- 

 pounds (nitrates, nitrites, ammonia salts), phos- 

 phates, calcium salts, and silicates. Nitrates and 

 phospates are particularly important as nutrient ma- 

 terial for phytoplankton. Calcium is required in large 



354 Geographic distribution of communities 



