THE ENVIRONMENT 



317 



littoral zone is that part of standing water where light 

 reaches the bottom. The limnetic zone is the illumi- 

 nated water above areas of ineffective light penetra- 

 tion. The projundal zone is the region of inefTective 

 light penetration. The benlhic zone is the bottom. 



TEMPERATURE LAYERING 



In deep standing waters (for practical purposes, 

 lakes) three characteristic temperature layers are 

 developed in the summer and the winter and lost in 

 the fall and the spring (Figure 17.10). These three 

 layers are from top to bottom the epilimnion, thermo- 

 cline, and hypolimnion. Often the epilimnion is an area 

 of warm water, the hypolimnion of cold water, and the 

 thermocline of rapid temperature change between the 

 other two areas. 



The annual cycle of lake waters that show tempera- 

 ture layering regularly displays the following gen- 

 eral pattern (Figure 17-11): 



In the spring the waters of the entire lake assume 

 the same temperature, and wind is the most impor- 

 tant force creating a circular path of movement from 

 top to bottom, the so-called spring overturn. As 

 summer temperatures rise, the surface waters be- 

 come warmer than the bottom waters and a thermo- 

 cline forms between the two. This stratification can 

 lead to hypolimnion oxygen depletion if the lake is 

 shallow and the hypolimnion is small, or if the 

 hypolimnion is below the depth of effective light 

 penetration. 



With the approach of fall, epilimnion temperature 

 gradually drops to that of the hypolimnion and the 



B 



Rgure ]7.10 Thermal stratification in lakes. Temperatures ( C.) are 

 given for 5-meter intervals. 



D E F 



Figure 17.11 The annual temperoture cycle in most deep-water lakes 

 of tennperote climates. (Temperature in degrees centigrade is given at 

 the surface and at depths of 5, 10, 15, and 20 meters.) A, the condition 

 of winter with a surface ice cover and lower stagnation; B, the time of 

 early spring when the surface waters ore wormer than those a few 

 meters down; C, spring overturn; the entire lake waters are 4'C. so 

 winds cause waters to overturn; D, summer stratification, the same condi- 

 tions shown in Figure 17.10, produced by surface worming; E, foil cool- 

 ing again causes surface waters to become cooler than intermediate 

 waters; F, fall overturn, like spring overturn, occurs when the entire 

 body of water is 4°C. 



jail overturn results. Further temperature drop takes 

 place but there is little change in condition until 

 4° C, the point of greatest water density. As the cool- 

 ing surface waters reach 4° C, they sink; cooling and 

 sinking finally causes the entire lake to obtain a tem- 

 perature of 4° C. When surface waters fall below this 

 temperature they become lighter and remain floating 

 upon the denser, deeper, 4° C. water. The conse- 

 quence again is stratification, but the warmer waters 

 are now in the hypolimnion and the surface of the 

 epilimnion usually freezes. This stratification tends 

 to cause litde oxygen depletion in the lake, for the 

 cold reduces the biological activities and oxygen 

 needs of its organisms. Actually, because they are 

 cold the waters may contain more oxygen than at any 

 other time of the year. The one general exception to 

 this is when deep ice or snow sufficiently retards light 

 penetration to prevent photosynthesis and to kill 

 many plants and, therefore, animals. 



