ZONES OF THE SEA-WHERE THEY LIVE 7 



are to understand the way animals and plants live. Light as we see it is actually 

 composed of a mixture of colors: red, orange, yellow, green, blue, indigo, and 

 violet. These spectral colors have been seen by anyone who has seen a rainbow 

 or looked at sunlight refracted through a prism. These colors take the form of 

 waves of varying length. As the length of the waves varies, so does the color 

 of the light vary. The longest visible wave lengths are red and the shortest 

 are \iolet. Light waves that are invisible to our eyes exist, and these are called 

 "infrared" (longer than red) or heat waves, and "ultraviolet" (shorter than 

 violet) or burning rays. 



Light has two important characteristics that are important to the photographer 

 (Chapter 3) and to the life of the sea. These must be distincdy separated in the 

 mind as two factors which may act either separately or together. They are 

 interisity and quality. 



Intcnsitv simply means brightness. Any wave length or combination of wave 

 lengths can exist over a wide range of intensities. As progress into the depth 

 of the sea is made, intensity of light decreases. Some animals have photoreceptors 

 that react solelv or chiefly to light intensity. Intensity regulates the behavior 

 of a great number of animals. Some plankton rises to the surface by night. 

 Squirrel fish, conchfish, coral polyps, and big-eyes become active at dusk. Plants 

 cannot photosvnthesize below a certain light intensity. 



Quality of light, or color, depends chiefly on differences in wave length from 

 red to violet. The color of an object depends on differential absorption of some 

 wave lengths of light and the reflection of others. For example, the squirrel 

 fish appears red because it absorbs all wave lengths of light except red, which 

 it reflects. Quality of light has important consequences for life. The different 

 groups of algae photosynthesize in response to different wave lengths. Adaptive 

 coloration of many animals is a response to light quality. 



It is now possible to descend from the surface to the abyssal depths of the 

 sea and to see what happens to light as progress is made downward. The most 

 immediate effect of sea water on light is reflection from the surface. In calm 

 waters about 3 to 4 per cent of light is reflected and never enters the sea, in 

 average waters 15 per cent, and in rough waters about 30 per cent. 



From the surface to depths, the quantity of light diminishes constantly due 

 to the opacity of water. It is possible to divide the sea into three zones on the 

 basis of light intensity alone. 



1. Euphotic or Well-Lit Zone. This is the shallow-water zone with a depth 

 range of about zero to 250 feet. It is rich in plankton, especially the 

 photosvnthetic types, and in herbivorous or plant-eating animals. Photo- 

 synthetic plankton is abundant to 150 feet and almost disappears at the 

 deepest part of this zone. The depth of this zone is largely determined 

 by the water's clarity. The Sargasso Sea is probably the clearest sea water 

 found anywhere. Sea water is generally more transparent than fresh water. 



2. Dysphotic or Dimly Lit Zone. From 250 feet to the level at which all 

 light disappears, the life of the sea is adapted to dim light. The level at 

 which all light disappears varies from 600 feet to about 5,600 feet depend- 



