Chap. I] 



AQUATIC ENVIRONMENT 



785 



of the year cease at 185 meters with a temperature of 15-6° C. The annual range of 

 temperature in the tropical zone of the Atlantic is 2-4 centigrade degrees, in the tem- 

 perate zone 7-2 degrees '. Shallow stretches of sea, and inland sheets of water, exhibit 

 greater variations than does the open sea. The following table gives the conditions 

 of temperature in a Central European lake : — 



TEMPERATURE OF LAKE PLON, ACCORDING TO ULE, 1892 (U), 

 AND APSTEIN, 1893 (A). 



In thermal springs the water in some cases (Japan, Mexico, South America, 

 Atlas) exceeds 90 C, but temperatures of springs or their effluents only need be 

 considered when below 60°. The flora of thermal springs, existing at a constantly 

 high temperature, contrasts with snow-flora and ice-flora, which exist at a constantly 

 low temperature. 



Light. The depth to which light penetrates water naturally depends on 

 the clearness of the water, and is therefore to a high degree affected by time 

 and place. Fol and Sarrasin found in Lake Geneva, in September, that 

 even at 170 meters a photographic plate became slightly dark, but at 120 

 meters very dark. In April, even at a depth of 250 meters, light was not 

 entirely absent. 



The different rays of the spectrum are very unequally absorbed in water, the 

 strongly refrangible rays from green to indigo less so than the weakly refrangible 

 rays in the red and yellow. Thus, according to Huffner, a column of pure water 

 180 centimeters long allows only 50 % of the red, but 90 % of the green, and 95 % 

 of the indigo, to pass. This inequality, on which the colour of water depends, 

 appears to be devoid of significance to plant-life. On the contrary, according to 

 experiments by Oltmann, the colour of the sea acts solely as a screen. 



1 Walther, op. cit., Introduction. 



3E 



