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 FitzG-erald, '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, aud 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- 



