592 TEXTBOOK OF BOTANY 



radiation from the sun and sky during a day, 7 per cent is reflected from 

 a smooth water surface; but when the water surface is rough, as much 

 as 25 per cent may be reflected. The hght below the water surface is 

 thus always less than that above. 



Natural bodies of water are usually colored or turbid with suspended 

 colloidal particles and microorganisms. These minute objects reflect 

 and refract the light in all directions. They also absorb it and are highly 

 effective in reducing light intensity in the water below them. About 40 

 per cent of the light that enters the water is usually absorbed in the 

 first meter. Not more than 5 to 10 per cent of the visible rays penetrate 

 to a depth of 10 meters; and at depths of 50 meters the light is negligi- 

 ble and consists onlv of certain green, blue, and violet rays. 



As a result of the reflection and absorption of light at the surface of 

 the water, the length of the daylight period in deep water is relatively 

 short and is limited to the middle of the day, when the sun's rays are 

 near the vertical. Radiation from the sky is insufiicient to illuminate the 

 depths. 



Photosynthesis. In clear- water lakes the layer of water in which effec- 

 tive photosynthesis occurs rarely exceeds a depth of 10 meters. In tur- 

 bid or colored water it is much less. It is tlierefore readily seen why 

 the bulk of the suspended and rooted plant populations of lakes is 

 confined to the upper layers of open water and shallow margins. Only 

 in the extremely clear water of high mountain lakes is photosynthesis 

 adequate at greater depths. There are a few very slow-growing green 

 plants that exist at depths of 15 and even 30 meters, where photosyn- 

 thesis in them is evidently just above the compensation point. The 

 maximum depth of most rooted aquatics in the lakes of the United States 

 is 6 to 7 meters. At this depth the plants receive about 2 per cent of 

 total sunlight. 



Oxvgen. Oxygen enters lake and pond water primarily from the at- 

 mosphere. It diffuses into the water more rapidly when the surface is 

 agitated and increased by wind. Additional dissolved oxygen is brought 

 into lakes from the atmosphere bv rain water and by rapid tributary 

 streams, particularly if they flow over falls and riffles. Oxygen is also 

 added during the daylight period by photosynthesis in submerged 

 green plants. 



The oxvgen concentration is decreased through the respiration of 

 both plants and animals. It is also dissipated by high temperature of 

 the water. Indeed, in shallow ponds containing an abundance of both 



