66 
BULLETIN OF THE BUREAU OF FISHERIES. 
These changes have been carefully studied on Lake Mendota, Wis. (Birge 
and Juday, 1911), and may be considered typical of most lakes of medium depth 
in the temperate zone; that is, where the winters are cold enough to cool the whole 
body of water below 4° C. or to freeze the surface. 
When the ice breaks up in the spring, the water below the ice usually has a 
temperature varying from 1 to 3° C. With lakes that do not freeze but have been 
cooled below 4° C. the spring circulation will start with the first warm days. In 
either case, as the surface is warmed the water increases in density until it reaches 
4° G. This denser warmer water settles and is replaced by the colder lighter water 
from below. This starts convection currents which circulate the whole lake until 
it reaches a temperature of 4° C., the temperature of water at maximum density. 
The strong winds that usually prevail at this season of the year materially aid this 
circulation. 
With all of the water at 4° C. and with the same density the whole body of 
water is kept in circulation by the wind. A strong wind will pile up the water on 
the lee shore. This water must return; part may return around the shore on the 
surface, but if the difference in temperature of the water is not over 2 or 3° C. part 
of the water will return along the bottom and so mix the whole lake. This is called 
the vernal or spring circulation. It usually continues until some time in May or 
June. During this circulation all of the water comes in contact with the air and 
is saturated, or nearly so, with oxygen. 
As the season advances the surface is warmed more rapidly, the winds are not 
as strong, and so the complete circulation is not continued. A part of the cold 
bottom water is not circulated by the wind. The surface water becomes warmer, 
therefore lighter and harder to mix with the colder denser bottom water. This 
difference in temperature and density increases until the lake is divided into three 
strata — the upper or circulated stratum, which is separated from the lower uncir- 
culated stratum by a shallow stratum, the thermocline, in which the temperature 
falls rapidly. As the season advances the circulated stratum becomes deeper and 
the thermocline more definite. Birge and Juday (1911) have called the stratum 
above the thermocline the epilimnion and that below, the hypolimnion. 
The epilimnion is circulated by the wind, so there is a very small variation of 
temperature from the surface to the lower part of the epilimnion. In the thermo- 
cline, which may be from 1 to 5 m. thick, the temperature drops very rapidly, usu- 
ally from 1 to 10° C. per m. In the hypolimnion there is a very gradual decrease 
in temperature from the thermocline to the bottom. During the remainder of the 
summer the hypolimnion is not circulated by the wind. It absorbs a small amount 
of heat through the thermocline, but the temperature of the hypolimnion increases 
very little during the summer. 
As the cool fall nights arrive the surface water is cooled, becomes denser than 
the warmer water, and settles. This continues until the difference in temperature 
between the surface and bottom waters is in the neighborhood of 3° C., when the 
difference in density is so small that the wind again takes an active part in the cir- 
culation of the whole lake. This is called the fall overturn and circulation. It 
continues until the lake reaches its minimum winter temperature or freezes. When 
the water reaches the temperature of 4° C., the convection currents set up by differ- 
