the (lc])tli to wliicli 1.0 i>er cent i)t surface light pene- 

 trates varies from 9.7 in when tnrbidity is 5 ppm, to 

 0.8 ni when turbidity is 115 ppm (Chandler 1942). 

 Since phytopiankton rec|uire light for photosynthesis, 

 abundance varies inversely with turbidity. Light 

 penetration is also affected by the abundance of or- 

 ganisms themselves, both phyto- and zooplankton. 



An appreciable amount of light jjasses through 

 ice in the winter. This enables phyto])lankton photo- 

 synthesis to continue. In eutrophic lakes many fish 

 may suffocate when snow overlies surface ice, pre- 

 venting photosynthesis and, thus, the generation of 

 oxygen (Greenbank 1945). 



The apparent color of water bodies may be the 

 result variously of sky reflections, the color of the 

 bottom, suspended materials, or of plants and animals. 

 Hut apart from these extraneous factors, water often 

 has an intrinsic color deriving from its chemical 

 contents. The blue color of pure water is a result of 

 blue light scattering by water molecules. Iron gives 

 water a yellow hue. A green color is usually associ- 

 ated with high concentrations of calcium carbonate. 

 \\'ater from bogs or swamps contains humic ma- 

 terials and is often dark brown. Many waters arc 

 essentially colorless. In a Wisconsin lake showing 

 practically no color, maximum photosynthesis of 

 algae occurred at one meter depth on bright days : 

 some photosynthesis occurred down as far as 15 

 meters. In a highly colored lake, maximum photo- 

 svnthesis occurred at 0.25 meter, none at 2 meters 

 r'Schomer 1934). 



Photosynthesis releases oxygen into the water : 

 respiration and decomposition absorb it. The upper 

 layer of a lake, where photosynthesis predominates, 

 is called the trophogcnic zone. Below this zone there 

 may still be considerable photosynthesis, but oxygen 

 absorption is greater than oxygen release. The 

 deeper portion of a lake is called the tropholytic zone. 

 The two zones are separated by a thin layer where 

 the oxygen gains from photosynthesis during the day- 

 light hours are balanced by the respiratory and de- 

 composition losses during the day and night. This is 

 the compensation depth, to which generally about one 

 per cent of the full sunlight at the water's surface 

 penetrates. The compensation level in a dark-colored 

 bog may lie less than a meter below the surface ; in 

 a deep, clear lake it may be 100 m down. 



Wind and currents 



^\'ind is an important environmental factor of 

 lakes because of water currents it generates. The 

 effect of wind action depends largely on the extent of 

 the exposed water surface, the presence or absence 

 of protecting upland, and the configuration of the lake 

 relative to the prevailing wind direction. 



Waves may become sizable in large lakes, but the 

 forward motion of a wave does not involve any great 

 mass of water. The rate of movement of surface 

 water is usually less than 5 per cent of the velocity 

 of the wind. The wave form moves on while the 

 water beneath undergoes a more nearly cycloidal mo- 

 tion, except along the shore, where the wave mass 

 progresses forward and breaks as surf. The water 

 washes back off the beach as an undertow, only to 

 he carried forward again by the incoming waves. 

 Tiie problem of maintaining position here is similar 

 to the problem of maintaining position in streams. The 

 depth of wave action in the open lake and along the 

 shore depends largely on the strength of the wind 

 (Kuttner 1953). 



In summer, surface water is warmed by solar 

 radiation and its density, weight, and viscosity de- 

 crease. In deep lakes the warm water piles up on the 

 exposed shore until, moving down along the bottom, 

 it encounters colder and denser waters, which resist 

 mixing. The warm water is then diverted horizon- 

 tally to the opposite shore. Thus the lake beomes 

 stratified horizontally into an upper cpilimnion, where 

 the water circulates and is fairly turbulent, and a 

 lower hypolnnnion. which is relatively undisturbed. 

 This difterence in circulation in deep lakes is closely 

 correlated with differences in temperature and oxy- 

 gen characteristics : it is of considerable importance 

 in the distribution of the biota. 



Temperatur(> 



The thermal conductivity of water is very low ; 

 but because of the thorough mixing of the waters in 

 the epilimnion during the summer by wind action, the 

 temperature is nearly uniform down to the thermo- 

 cline. The thermocline is the zone of most rapid 

 temperature decrease, generally involving a drop of 



FIG. 6-2 Water cu 

 Uke. 



Lakes 61 



