COMMUNITIES IN OTHER INLAND WATERS 363 



predominate in shallow lakes of melted snow and ice; they grow to an 

 astonishing size in a few weeks. Nevertheless L. arcturus requires at 

 least V/2 months from egg to egg, so that this species is absent in the 

 coldest regions. 19 The winter eggs must freeze before they can de- 

 velop.— 



In spite of the small number of species life in a polar lake 

 may be very plentiful under favorable conditions. It is extraordinarily 

 rich in Lake Myvatn 20 in Iceland. The fine bottom mud is inhabited 

 by immense numbers of Chironomus larvae, and the larvae of mos- 

 quitoes and black flies are plentiful. A large number of entomostracans, 

 especially cladocerans, live in the pedonic region; on the other hand, 

 because of the shallowness of the lake, there is only one species of 

 limnetic crustacean, Daphnia longispina. Where isolated warm springs 

 arise, the number of snails (Limnaea, Planorbis) is very large. In 

 correlation with this rich supply of food the lake contains large num- 

 bers of trout. 



In winter the covering of ice and the masses of snow settling on it, 

 both poor conductors of heat, form a protection against too rapid 

 cooling. Thus even the shallow Lake Sadonach near Werchojansk in 

 Siberia, which is only a little more than 3 m. deep in the deepest 

 places, never freezes down to the bottom, in spite of the extreme cold, 

 and in Spitzbergen bodies of water more than 2 m. deep do not freeze 

 solid. Moreover, many aquatic animals can stand freezing without 

 harm. The winter eggs of crustaceans survive being frozen into the ice; 

 rotifers also are insensitive to freezing, whether as eggs or adults. 

 Rotifers which were frozen into a piece of ice 5 m. below the surface 

 can remain unharmed for several years. 23 Nordenskjold 24 found 22 

 species of mollusks* frozen in the ice, and after careful thawing nearly 

 all the animals proved to be alive. There is, however, only one water 

 snail, Limnaea peregra, which ranges beyond the Arctic Circle in Nor- 

 way. 



The lakes of the Scandinavian highlands and the lakes of high 

 mountains 25 at a height greater than 1500 m. above sea level show 

 a striking similarity to polar waters. These lakes are free from ice 

 for only a short period of the year; Lake Katrak, in Torne, Lapland 

 (776 m. above sea level), only for 2-2V> months; and lakes lying 

 even higher up in the Sarek highlands are free from ice only for a 

 few weeks in warm summers. Lake Partnun in Switzerland is ice-free 

 for 5V2 months, and Liiner Lake for 6 months. The temperature at the 

 surface hardly rises above that near the bottom of the lakes in the 



* Limnaea, Planorbis, Amphipeplea, Physa, Bythinia, Valvata, and Pisidium. 



