PHYSICAL CONDITIONS IN THE OCEAN 161 



the water, is quite different from that in air, and the deeper the water, 

 the greater is the amount of this difference. The different wave lengths 

 which compose ordinary light are differentially absorbed by the water, 

 the long wave lengths much more rapidly than the short. At 100 m. 

 all the colors are still present, but the red rays are most sparingly 

 represented, the green more abundantly, while blue and violet pre- 

 dominate. At 500 m. the blue to ultra-violet rays are still abundant, 

 while even 40 minutes' exposure of suitably prepared plates fails to 

 discover a trace of red or green. The strong absorption of the red rays 

 explains why red coloring matter serves so frequently for color pro- 

 tection in the greater depths. Beebe, in his bathysphere dives, de- 

 tected no violet with his eye, but only a strange blue which appeared 

 brilliant even after losing power to illuminate objects. 



The transparency of the sea water also depends on the amount of 

 material in suspension, whether organic or inorganic, and thus varies 

 with location, according to the presence of large amounts of plankton 

 or of finely divided silt, as at river mouths. The depth at which light 

 is reduced to 1% of its surface value is found at 8 m. in the Woods 

 Hole Harbor, at 32 m. in the Gulf of Maine, and at 149 m. in the 

 transparent waters of the Sargasso Sea. The transparency of the 

 Sargasso Sea is the greatest yet measured, with the possible exception 

 of the Mediterranean. The transparency decreases greatly on the con- 

 tinental shelf. The relative opacity of fresh waters is better realized 

 when it is known that the clearest yet measured, Crystal Lake, Wiscon- 

 sin, is approximately equal in transparency to the water of Vineyard 

 Sound, which in turn approaches that of Woods Hole Harbor. 62 - 6G 



The differential absorption of light exerts an important effect on 

 the distribution of the plant life and its dependent animal life. It is 

 precisely the red, orange, and yellow rays which are most important 

 in photosynthesis, and these are present in sufficient amounts only in 

 the uppermost levels. The plant distribution is accordingly very un- 

 equal at different depths. The maximum development of plant plankton 

 comes between depths of 20 and 50 m., and the maximum is ap- 

 proached from the depth of 10 m.; at 75 m. there is only one-half of 

 the amount at 50 m., and at 100 m., only one-fifth. The reduction 

 of the amount of plant life with depth occurs still more rapidly in 

 polar waters, where the light extends to a lesser depth. The same 

 effect, in a still higher degree, is notable in coastal waters, where 

 light is excluded by turbidity. Seaweeds and algae scarcely extend 

 below 40-50 m., on the Danish coast, and eelgrass (Zostera) only to 

 14 m., in most transparent coastal waters, while in the fiords of Jut- 

 land, rich in plankton, it extends only to a depth of 4-5 m. 



