290 
Birge—The Crustacea of the Plankton. 
It is, of course, possible that the lake should freeze when the 
bottom is at any temperature between 4° and zero. It is hardly 
probable, however, that it often freezes permanently when the 
bottom is lower than 1° or higher than 2.5°. Below the ice the 
temperature of the water rises rapidly, being half a degree or 
even more within less than half a meter of the ice, and below 
this level the temperature rises very slowly and regularly to the 
bottom of the lake, the difference between the water at 0.5 m. 
and the bottom rarely exceeding two degrees. The mud is or¬ 
dinarily decidedly higher in temperature than the water just 
above it. (See FitzGerald, ’95, p. 81.) The difference between 
the temperature of the mud and the water half a meter from the 
bottom was sometimes found to be as great as 0.7-0.9° in 
1894-5, and 1895-6, by the aid of the water bottle; while the 
thermophone in 1897 showed differences of 0.3-0.8°. This dif¬ 
ference varies in different parts of the lake without any assign¬ 
able reason. 
The temperature of the water of the lake rises during the 
winter, especially during the latter part of February and March 
(Cf. Apstein, ’96, p. 18). In 1895 the temperature reached 
nearly 2.5° at the bottom, and 1.5° close to the ice on the 27th 
of March. In 1896, on March 28th, the temperature at one- 
half meter was 2.9°, at the bottom (18 meters) 3.1°. This was 
a rise of from 1.5 to 2° during the winter. In 1897, the tem¬ 
perature on January 23rd was: 1 m., 0.6°; 18 m., 1.8°, On 
March 29th, at 1 m. the temperature was 1.4°, at 18 m., 2.1°. 
This warming of the water is due to the sun. If it were due to 
warm water coming from springs the bottom temperature would 
necessarily rise to 4° before the change appeared in the upper 
water. But this is not the case. The temperature at the bot¬ 
tom has not reached 4°, in any of the three winters during 
which observations have been taken, until after the breaking up 
of the ice in the spring. It would appear, therefore, that this 
warming must be due to heat which enters the water from above. 
While this rise in temperature is very gradual and is small in 
amount, it has important biological results. The reproduction of 
Cyclops and of the rotifers goes on very much more rapidly at 
a temperature above 1.5° than at a temperature near 1°. In- 
