NORTHWESTERN- LAKES OF THE UNITED STATES. 
67 
ence in density of the warm and cold water cease to aid the circulation. Water 
cooled below 4° C. becomes lighter and tends to stay on the surface, but this differ-, 
ence in density is not enough to prevent the wind from continuing the circulation. 
This circulation will continue all winter if the lake is not cooled until it freezes. 
If the temperature of the water drops to 1°, a cold, still night will cool the surface 
until it freezes. 
When a lake freezes, the water below the ice has a temperature below 1° C. 
from the ice to the bottom of the lake. During the winter the lake lies dormant. 
There is very little circulation. The oxygen that the water dissolved from the 
air during the fall overturn is gradually used up by the slow decay of organic matter 
and the respiration of the animal life. Both of these processes go on much more 
slowly in winter than in summer. 
The comparison of the dissolved gases and thermal conditions of these lakes 
is simplified by arranging them in groups that will bring similar lakes together. 
A classification suggested by Birge and used in comparing the Wisconsin lakes is 
based upon the thermal and gas conditions. Group I includes the lakes that have 
the entire body of water kept in circulation during the summer; that is, they have 
simil ar gas and thermal conditions from surface to bottom. Generally these lakes 
are shallow, but in a few cases large lakes or those exposed to strong winds are 
completely circulated where they have greater depth than smaller protected lakes, 
which fall in the next group. 
Group II is composed of those lakes that are not completely circulated during 
the summer. In such lakes the three strata — epilimnion, thermocline, and hypo- 
limnion — are found during the summer. The supply of oxygen in the hypolimnion 
is-limited to that which it absorbed during the spring or vernal circulation. During 
the summer dead animal and vegetable material, mostly plankton, settles into 
this lower water and decays. This uses up the oxygen and increases the carbon 
dioxide in the water, and the respiration of the fish and plankton Crustacea uses 
an appreciable quantity of oxygen. If the hypolimnion has a small volume and 
there is a large amount of decomposition, the oxygen of the bottom water may 
be entirely exhausted during the late summer. On the other hand, if there is less 
decomposable material, a larger volume of water in the hypolimnion, or a shorter 
season, the oxygen may persist during the whole season. These facts have been 
used by Birge to make two divisions of the second group of lakes. The first division 
contains those lakes whose bottom water contains some oxygen during the whole 
summer. The second division is composed of the lakes in which the oxygen of the 
bottom water is entirely used up during the summer. 
In Group I there are seven lakes (Henry, Green, Upper Twin, Steilacoom, 
Liberty, Fish Trap, and Upper Klamath), which vary in depth from 2 to 11 m. 
Henry, Green, and Fish Trap are ideal examples of this group. There is little or 
no variation of temperature or dissolved gases from surface to bottom, thus showing 
complete circulation of the whole body of water. The others show some change 
in thermal and gas conditions. Where there is a lower bottom temperature it indi- 
cates incomplete circulation, but in no case does the oxygen disappear at the bottom, 
which would probably be the case if the lower water was entirely cut off from the 
upper by a thermocline. Upper Klamath shows a marked increase in temperature 
