LIMNOLOGICAL PROBLEMS 
393 
The height of the water undergoes very considerable variations in the 
course of the year, especially where the lake is fed from melting snow ; 
the renewal of the water proceeds unequally at the various seasons — 
most rapidly in spring, when immense quantities of water during thaw 
pour into the lakes. Owing to the great evaporation in summer and 
the decrease in all affluents the level of the water sinks greatly ; the 
degree of concentration combined with great but regular variations 
in transparency, colour of water, etc., therefore uiidergoes consider- 
able oscillations. On the great changes in level in the high alpine 
lakes see Zschokke (1900, p. 17). 
Information regarding the temperature in many lakes of this zone is 
to be found in numerous records of many authors. It is not my inten- 
tion to give here a summary of our knowledge of lake temperatures in 
general, but merely to emphasise those features which are characteristic 
of the lakes in each zone. After having studied this literature, it has, 
however, been impossible for me, apart from the little advanced here, 
to discover any features which might be said to characterise the alpine 
lakes in contrast to the Baltic lakes. 
Temperature varies greatly, of course, but presents, on the other 
hand, a certain amount of uniformity hitherto hardly sufficiently 
noticed. There are lakes w^hich must be designated as completely 
arctic, frozen even in the middle of sunnner or with masses of ice 
floating on their surface and the sunnner temperature hardly exceed- 
ing 2-3^ C. Such lakes are mentioned by Monti : Lac de Seracs (at a 
height of 2370 m. ; the surface was in September covered w ith ice, and 
the temperature at surface w as only 2° C. ; 1906, p. 131), Lac de Grand- 
Domenon in the massif of Belledonne (Delebecque, 1898a, p. 170), 
Lac d'Arrius (Delebecque, 1898a, p. 171). The lake of St Beiiiard 
hospice, at a height of 2445 m., is closed up in certain years for 330 
days: it closed on the 22nd October and was not open till the 15th 
September (Zschokke, 1900, p. 35). The majority are no doubt tem- 
perate lakes, but approach the arctic type more or less : there are 
lakes which one year may be designated as arctic, in others as tem- 
perate. Concerning all these lakes there is much extremely interesting 
information in Zschokke's excellent chapter on temperature in high 
alpine lakes (1900, p. 20). In the same zone in which we find these 
lakes, situated under more or less arctic conditions, the temperature 
of which at any rate in certain years does not exceed 4° C.„we find 
distinctly tropical lakes which never freeze — Lac Leman (Forel, vol. ii. 
p. 395) and the North Italian lakes — or only exceptionally — Bodensee, 
seven times since the year 1227 (Geistbeck, 1884-5, p. 364) — or only ex- 
ceptionally and in part — Vierwaldstiittersee (B. Amberg, 1904, p. 142). 
How^ever nuich all the lakes of this zone difler in regard to winter 
temperatures and ice conditions, their sunnner temperatures are some- 
