1904 - 5 .] Study of the Lakes of Scotland and Denmark. 417 
phycea — except Oscillatoria — usually reach their greatest develop- 
ment at the highest summer temperature (between 19° and 23° C. 
= 66° and 73° F.) and in the warmer lakes. Accordingly, we find 
a great development of Diatoms in the cold northern lakes as well 
as in the southern alpine lakes, and an almost complete absence of 
the Cyanophycea in both these localities, the only exception being 
the Oscillatoria and partly Anabcena jflos aquoe, which are both 
common in the alpine lakes of Switzerland. In our colder lakes 
a great development of Diatoms occurs in the last days of April, 
when the lakes are ice-free, and continues till June ; then a great 
development of Ceratium liirundinella sets in, and in September 
a second development of Diatoms appears. On the other hand, 
in our shallower and warmer lakes the great development of 
Diatoms is discontinued a little earlier, then the Cyanophycea 
appear, and often predominate throughout the rest of the year ; 
still, in these lakes also the development of Ceratium and a second 
development of Diatoms occur, but rarely to such an extent as in 
the deeper and colder lakes. 
The deep cold lakes rarely present the phenomenon of “ wasser- 
bliithe ” ; and if it appear, it is only for a short time in June, caused 
by Anaboena flos aquae. As the chromatophores of the Diatoms, as 
well as those of Ceratium liirundinella , are a yellowish-green, the 
colour of the water in nearly all our colder lakes is also yellow- 
green. The colour of the water in the shallower and warmer 
lakes is in spring also yellow-green, owing to the first great 
development of Diatoms ; but when the maximum development of 
the Cyanophycea sets in, the colour becomes more bluish-green, 
owing to the blue-green colour of the Cyanophycea cells, and the 
surface of the water on calm days is covered by a thick layer of 
“ wasserbliitlie ” : in August and September, when the great 
development of Cyanophycea is intermixed with that of Ceratium 
liirundinella and the second development of Diatoms, the water in 
these lakes changes somewhat towards yellow-green. 
As a rule, we may say that the colour of the water in our lakes 
throughout the greater part of the year is determined by the 
colour of the plankton-organisms, especially by that of the 
chromatophores of the Diatoms and of the Cyanophycea. Only in 
April, immediately after the breaking up of the ice, is the quantity 
PROC. ROY. SOC. EDIN. — YOL. XXV. 27 
