PHYSICAL CONDITIONS IN THE OCEAN 



155 



Temperature. — The temperature relations of the water are of 

 maximum importance to marine zoogeography. The temperature of 

 the sea water varies not only with location but also with the seasons, 

 in different degrees. The polar seas may fall below 0°, and in the 

 tropics the surface water may reach a temperature of 30° or more 

 (34° in the Red Sea in summer) . The regional temperature differences 

 are of importance as barriers and as impetus for variation. The ampli- 

 tude of seasonal variation in temperature depends on the location. The 

 temperatures of the tropical and polar seas are relatively uniform, 

 while the seasonal variation is high in the temperate zones. The annual 

 variation in temperature is less than 5° on almost three-quarters of 



Fig. 12.— Variation of the surface temperature in the open ocean off the 



west coast of Scotland ( ) and in the stagnant Baltic Sea ( ). After Meyer 



and Mobius. 



the oceanic surface, and on one-third of this (especially in the tropics) 

 less than 2°. The greatest variations appear in localities where warm 

 and cold currents meet and predominate by turns. The variations are 

 also high in arms of the sea in the higher latitudes. The annual vari- 

 ation in the northern Mediterranean reaches 14°, in the Baltic 17°, 

 in the inner parts of the Yellow Sea 27°. Such seasonal variations 

 apply only to the surface layers, ceasing at depths of about 200 m. 

 Coastal areas have a higher temperature variation than the open sea, 

 and the temperature amplitude is also large in shallow seas, as in the 

 Baltic (Fig. 12). 



A stratification of temperatures is a result of the property of water 

 whereby it increases in density as the temperature drops. The heavy 

 cold polar water sinks and gradually spreads out on the bottom toward 

 the equator, and the lighter warmer water of the tropics flows toward 

 the poles. The radiant heat of the sun does not reach deep into the 



