March 9, 1900.] 



jSCIENCK 



381 



infer that the whole plankton moved up 

 and down. 



The limits of this vertical migration it is 

 very difficult if not impossible to fix. Most 

 of the movement is within one meter of the 

 surface, the most marked changes being 

 within one-half meter of the surface, and 

 below three meters the amount of move- 

 ment is very slight. Eight determining 

 factors have been listed by Professor Birge 

 as controlling the vertical distribution of 

 Crustacea : food, temperature, condition of 

 the water in respect to dissolved oxj'gen 

 and other substances, light, wind, gravity, 

 age and specific peculiarities. Of these 

 factors, by far the most important are food, 

 temperature and light. Inasmuch as the 

 food supply is controlled by temperature 

 and light, we may speak of these two fac- 

 tors as, in the main, controlling the vertical 

 distribution of the limnetic plankton. Of 

 these two factors, temperature is the most 

 important, although light has a marked 

 eflfect on many species. In the winter 

 season when the waters of all the lakes are 

 very nearly uniform in their temperature 

 from top to bottom, the vertical distribu- 

 tion of the limnetic fauna is much more 

 uniform than in summer. 



In the summer season the most marked 

 changes in vertical distribution are corre- 

 lated with the vertical changes in tempera- 

 ture. This is most distinctly seen in the 

 deeper lakes. In these lakes it is a surface 

 layer of greater or less depth which is 

 warmed, the deeper layers feeling the effect 

 of the summer's sun only very slightlJ^ In 

 Green Lake below 40 or 45 meters the tem- 

 perature never rises above 6.11 degrees 

 Centigrade, although the surface may run 

 as high as 26.67 degrees C. In Lake 

 Michigan the bottom temperature at depths 

 of 360 feet is 4.2 degrees C, with a surface 

 temperature of 18.3 C. 



The change in the temperature from the 

 top to the bottom is not a gradually decreas- 



ing one, however. A layer of water at the 

 surface, which may be in midsummer some 

 ten or twelve meters in depth, is very nearly 

 uniform in temperature. From the lower 

 surface of this layer there is a very rapid de- 

 crease in temperature for a short distance, 

 and then a gradual decrease until a minimum 

 Is reached. This layer of sudden change in 

 temperature is known as the ' thermocline,' 

 and its position varies in depth with the 

 season and the size of the lake. As the 

 summer season progresses the thermocline 

 grows lower. In the very shallow lakes the 

 temperature during the summer season is 

 nearly uniform through the whole depth. 

 In Lake Winnebago, for example, there is 

 seldom a difference between top and bottom 

 temperatures greater than two degrees. In 

 small lakes the thermocline is considerably 

 higher than in large lakes. This is doubt- 

 less due to the influence of the winds, by 

 which the small lake is less affected. This 

 was very prettily illustrated in a compar- 

 ison of the Waupaca lakes with Cedar Lake 

 and Green Lake about August 1 st of this last 

 summer. In thi'ee of the Waupaca lakes — 

 Eainbow, McCrossen and Beasley's, of which 

 Eainbow is the largest and Beasley's the 

 smallest, the thermocline was respectively 

 at six meters, five meters and three meters. 

 At the same time the thermocline of Cedar 

 Lake was at eight meters, and the thermo- 

 cline of Green Lake at eleven meters. The 

 vertical distribution of the plankton has a 

 very close relation to the thermocline, most 

 of the animals being above it. Limnoca- 

 laiius, Daphnia longiremis and Daphnia puli- 

 caria, however, are found below the thermo- 

 cline, and in some plankton-poor lakes the 

 proportion of the other organisms below the 

 thermocline is much larger. 



It is evident that the circulation of the 

 water is in the layer above the thermocline, 

 and that below the thermocline there is in- 

 sufficient oxygenation, and that this bottom 

 layer must, too, hold a great deal of the 



