388 THE FRESH-WATER LOCHS OF SCOTLAND 
at the margin of the ice, out towards the open water, 3° C. The 
temperature of the ground on the edge of the water, on the sunny 
side, was 12° C. 
A continuous series of warm, bright days at the end of March 
(10-11° C. at midday) and in the beginning of April raised the 
temperature of the pelagic region in the small ponds unusually high 
(6-8^ C), whilst during the same period the temperature in the pelagic 
region of Fureso rose only 1° — from 2°'l to 3°*1 in the course of four 
days, 28th to 31st March. In the following three weeks, when the 
temperature of the air never rose above 5 '6 C. and was generally 
lower, the temperature of the water in the ponds nevertheless steadily 
rose to 6° '7 C. ; the surface temperature in the pelagic region of 
Fureso rose immediately after the above-mentioned warm days (28th 
to 31st March) to 4° C. and continued to rise to 5° "7, at which tempera- 
ture the surface of the lake remained to the last days of May. These 
high water temperatures cannot possibly be due to the warmth of 
the sun at that time, as the air temperature was throughout lower 
than the water temperature. In my opinion, it was in the first 
instance the high temperatures occurring in the littoral region in 
very early spring which imparted a surplus of warmth to the pelagic 
regions of the lakes and ponds, a warmth retained later ; and 
further, it was the warmth-collecting quality of the littoral region 
which absorbed every sunbeam cast upon it in the foggy, rainy 
April, that was later of benefit to the pelagic region. Various 
things seem to me to indicate that under our climatic conditions the 
monthly average temperature of the water in the summer months 
in the pelagic region of our lakes will be above the average tempera- 
ture of the air. That, on the other hand, the littoral region becomes 
extremely cold during the period of cooling is a well-known fact 
which I need not discuss here. As I have the impression, however, 
that the importance of the littoral region as a store of warmth, at 
least in the Baltic lakes, is less well known, I have dwelt upon the 
subject here at some length. 
What also acts in these lakes as a warmth-absorber, and, under 
the influence of the processes of putrefaction, as a warmth-producer, 
is the enormous quantity of plankton, especially during the periods of 
the water-bloom, which is produced in the lakes in the warm summer 
months. We have unfortunately no data to show how high these 
temperatures may rise. 
Further, the rich organic life influences the colour of the water 
in the Baltic lakes ; the true colour has, so to speak, never been 
seen, being almost always determined by the plankton, that is, by 
the particular coloured bodies in the plankton organisms which have 
their maximum at the moment in the lake (on this point see especially 
