LIMNOLOGICAl. PROBLEMS 
381 
In temperate lakes it will generally appear in June, and sink deeper 
and deeper in the course of summer until it reaches its deepest point 
in autumn and then disappears during the cooling processes. 
It appears from the researches in Mjosen, Wetter, and Ladoga 
that the " Sprungschicht " is at its deepest point, at about m., 
in the beginning of September ; the change in temperature may here 
amount to 3-5" C. Quite different phenomena have been observed 
by Wedderburn (1907a, p. 407 ; 1907b, p. 1) in Loch Ness. 
I am inclined to believe that a great many northern temperate 
lakes, especially those of the tropical type, are most probably 
characterised by the deep-lying position of the " Sprungschicht," or, 
in other words, by the great thickness of the layer of water where the 
temperature is uniform. The reason is perhaps partly the small 
quantities of plankton in these northern temperate lakes, partly the 
effect of wind upon water-basins of an almost always very elongated 
form, though principally the mild winters in the western parts of 
the zone. 
It is clear that the cuiTcnts in these lakes are much stronger than 
in the polar lakes, a fact which is of very great importance for the 
migration of the plankton. The seiches have been studied by 
Holmsen (1898, p. 1) in Norwegian lakes, but especially by Chrystal 
(1905a, p. 599 ; 1905b, p. 637) and Chrystal and Wedderburn (1905, 
p. 823) in Scottish lakes. 
Of the vegetation in the lakes and its arrangement in zones 
we know exceedingly little. The vegetation in the Scottish lakes 
was studied by G. West (1905, p. 967), that of the Faroes by 
Ostenfeld (1906, p. 62). From Iceland, Norway, and the northern 
parts of Sweden and Finland we have very little information. The 
vegetation in the lakes seems always to be very slight ; we do not 
know to what depth the wholly submerged vegetation belts of 
Characece and Font'inalis penetrate ; in the Scottish lakes it is 
commonly not great, according to West, on account of the dark, 
peaty water. A remarkable difference between the lakes in this and 
the following zone is that the Potamogeton belt is not very distinct ; 
the belt of Phragmites and Sciiyus so characteristic of the Baltic lakes 
is either weakly developed or quite absent. The low temperature, 
the shores as a rule steep and covered with rolling stones, the wave 
erosion, the slight detritus formation, the fact that the water is 
usually slightly transparent and the percentage of lime generally 
small, are all instrumental in causing the vegetation belts of the 
North European lakes to be weakly developed. 
In localities where the transparency is greater, the percentage of 
lime in the water great, and where the shores are evenly sloping, we 
find lakes where the vegetation belts are as broad as those of the 
