322 ANIMALS IN INLAND WATERS 



the greatest yield of fish (113 kg. per hectare) ; the yield of the largest 

 lakes (Lake Vener, 566, 800 hectares; Lake Malar 116, 200 hectares) 

 is much smaller and amounts to only 1.4 kg. and 2.7 kg. respectively, 

 per hectare. The relatively largest amounts of living forms are found 

 in the small ponds (village ponds, etc.), which are richly fertilized. 



Temperature relations. — Temperature is of special importance 

 for life in fresh waters. In consequence of its high degree of fluidity, 

 the water becomes stratified according to density. As stated in Chapter 

 XVI, the density of fresh water increases with a decrease in tempera- 

 ture until it has reached its maximum at 4°, and then with a further 

 lowering of temperature the water expands again and becomes lighter. 

 The cooling of water occurs by radiation of warmth from the surface ; 

 consequently the surface layers become denser and sink until they 

 reach a layer of equal temperature, and their place is taken by the 

 warmer masses of stagnant water which rise to the top only to sink 

 again when they become cooled. After long-extended cooling the whole 

 mass of water reaches a uniform temperature of 4°. Further cooling of 

 the surface water causes it to expand again and become lighter; this 

 cooler layer remains on top, and finally, when a covering of ice hinders 

 any further giving off of heat, the cooling spreads but slowly to the 

 deeper regions because of the poor thermal conductivity of water. On 

 the other hand, when cold surface water which has reached a tempera- 

 ture below 4° is warmed, the upper layers become denser, because of 

 the change from perhaps 0° to 1°, and sink, and this process continues 

 until the whole water mass has again acquired uniformity of tempera- 

 ture at 4°. Further warming then penetrates the lower layers very 

 slowly. During the season when the surface is warmer than 4° the 

 temperature decreases with increase in depth; this is called direct 

 stratification. When the surface is colder than 4°, the temperature rises 

 with increasing depth; this is inverse stratification (Fig. 92). 



The convection currents set up by the cooling and warming of 

 water are of great importance in supplying the deep water with 

 oxygen. They occur most regularly where the greatest changes of 

 temperature occur, viz., in the temperate zones; they are much re- 

 duced in the polar regions, on the one hand, and in the tropical or 

 subtropical regions on the other. 



Thermal conditions in standing waters are very strongly influenced 

 by the change of seasons in the temperate zones. At a given time in 

 autumn all the water has a temperature of 4°; if it continues to cool 

 at the surface, an inverse stratification sets in without creating con- 

 vection currents, i.e., winter stagnation. Then warming begins, which 



