284 THE FRESH-WATER LOCHS OF SCOTLAND 
the Algae did not form streaks or patches on the surface, but could 
be seen floating scattered through the water. 
A distinct " flowering due to Chlorophjcese has been seen in a 
shallow loch as early as July. The phenomenon was commoner in 
the autumn, in August, and especially in September, among the 
northern lochs, and was generally caused by Myxophyceae {Anab(Fna, 
Oscillatoria^ etc.). 
In contrast to this summer and autunni " flowering,"' some lochs 
of considerable size (Loch Earn, St Mary's Loch) flowered in mid- 
winter, when the water was coolest. These lochs had clear water in 
summer when the Algae were scarce, but became more or less turbid 
from the abundance of vegetable life in winter. 
Temperature in relation to Plankton. — In respect of the tempera- 
ture of the water, as afl'ecting the biology, Scottish lochs fall into 
two classes — those which do not freeze in winter, and those which 
do freeze. The climate of Scotland is so temperate that no 
lochs are in normal winters frozen over for long periods, as they 
are in Europe generally. The smaller lochs may be frozen over 
for a few days, or a few weeks, several times in the course of the 
winter. The distinction between the two classes of lakes is, however, 
quite sharp. 
The first class contains lochs of such a size that an ordinary 
winter is not sufficiently long to cool them below the maximum 
density point of water. Their minimum temperature is therefore 
nearly 40"" Fahr., and the larger lakes seldom cool to within several 
degrees of this. The volume of water which prevents cooling below 
this point in an ordinary winter also prevents undue heating in 
summer, and the characteristic of these lakes is the small annual range 
of temperature, rarely reaching 20° "0 Fahr. This very temperate 
climate must be of great importance to the inhabitants of the lakes, 
and might be expected to reduce seasonal change to a minimum. 
The second class contains lakes which cool sufficiently in an 
ordinary winter to cause them to freeze from time to time. The 
minimum temperature of these lakes may be anything down to freezing 
point, but it is rarely that anything less than S5°*0 is recorded in the 
water beneath the ice. 
A temperature of 35°"0 has only been noted in very shallow lochs, 
and more usually, when the ice is broken over fairly deep water, a 
temperature of just about maximum density point is found. Lochs 
shallow enough to freeze in winter are also likely to become greatly 
heated in summer. These lochs have therefore a much higher annual 
range of temperature. It is rarely that any Scottish loch reaches a 
higher temperature than 64° *0 Fahr., so that the extreme range even 
for small lochs is only about 30°'0 — very much less than the range in 
