As tlie ice melts during the sjiriiig aiul the sur- 

 face waters warm up, a sf>rin(i overturn occurs wlien 

 the water at all depths is at the same temperature. 

 The time and duration of the spring overturn de- 

 pends on weather conditions ; it may last several 

 weeks. It often occurs intermittently, however, cor- 

 responding with changes in weather and water tem- 

 peratures. 



When a lake has two overturns during the year, 

 it is called a diiitictic lalcc. Such lakes are character- 

 istic of. but not limited to, temperate climates. In 

 warm, oceanic climates and in the tropics, the sur- 

 face waters may not cool sufficiently to permit com- 

 plete circulation, e.xcept during the coldest period of 

 winter. Lakes undergoing a single overturn are 

 called ti-anii monomictic. The temperature of the 

 water in the hypolimnion of such a lake is never 

 lower, of course, than the mean air temperature dur- 

 ing the period of the last complete circulation ; in 

 w-arm climates this may be several degrees above 

 4°C. On the other hand, lakes in polar or alpine 

 regions may never warm above 4°C, and complete 

 circulation occurs only in the middle of the summer. 

 These are cold monomictic lakes (Hutchinson 1957). 

 The three types of lakes were formerly called tem- 

 perate, tropical, and polar, but this terminology is 

 undesirable since their geographical segregation is 

 not precise. 



Lakes of the first order are those in which the 

 bottom water remains at or near 4°C throughout the 

 year, and while one or two circulation periods are 

 possible, there is often none. In lakes of the second 

 order, the temperature of the bottom water rises 

 above 4°C during the summer, and there are one or 

 two regular circulation periods during the year. 

 Lakes of the third order do not develop thermal strati- 

 fication, and circulation of water is more or less con- 

 tinuous (Whipple 1927). In general, lakes over 90 

 meters in depth belong to the first order : those be- 

 tween about 8 and 90 meters belong to the second 

 order : and those less than 8 meters to the third order. 



The specific heat of water is greater than most 

 other substances : accordingly, a vast amount of heat 

 must be absorbed to cause a temperature change. 

 Temperature change is, in any event, slow. Much of 

 the energy of solar radiation is lost by reflection from 

 the water surface. The rest of the radiation is ab- 

 sorbed by the water, the solutes, and the suspended 

 material. But much of the diurnal energy increment 

 may be dissipated by re-radiation at night or in 

 cloudy weather, by evaporation, and by convectional 

 cooling. The amount of heat actually retained by a 

 lake to melt its winter ice and warm it from the 

 winter minimum up to the summer maximum is its 

 annual heat budget (Table 6-1). For many dimictic 

 lakes this is between 20,000 and 40,000 g-cal/m- of 

 surface ; there is wide variation in different kinds of 



TABLE 6 I Monthly change in cumuUtivs heat budget and solar 

 radiation In the Bass Islands region (depth 7.5 m| of western 

 Lake Erie, in 1941. A 20.3 cm ice covering formed in mid-Janu- 

 ary, melted in late March. The maximum heat budget, reached 

 on July 30. was 19,575 g-cal/cm*. The heat budget was about 

 15 per cent of the total solar radiation received during the 

 year (after Chandler 1944). 



lakes. The annual heat budget is important in de- 

 termining a lake's productivity. 



OXY 



gen 



The distribution of o.xygen at various depths 

 depends upon the presence or absence of a thermo- 

 cline, the amount of vegetation, and the organic na- 

 ture of the bottom. The amount of oxygen in water 

 is only one-fortieth to one-twentieth of that present 

 in an equal volume of air when the two are at equi- 

 librium, although their partial pressures are the same. 

 Diffusion of oxygen from the air into comparatively 

 sedentary water occurs very slowly ; agitation of the 

 water increases the surface area and promotes a faster 

 rate of equilibration. 



SAMPLE DATES- "^^^"""^ "^ '^ 



12 3 4 5 6 7 



CUBIC CENTIMETERS PER LITER 



FIG. 6-4 Changes in the vertical distribution of oxygen through- 

 out a year in a dimictic eutrophic lake — Lake Mendota, Wis- 

 consin (after Blrge and Juday 1911). 



Lakes 63 



