The Distribution at the Thermocline. 417 



The distribution of the nauplii at the thermocline is especially- 

 noteworthy. During the period of the observations there were 

 frequently found enormous numbers of larval Copepods in the 

 lower water. The numbers began to increase at ten or even 

 eight meters, at a point several meters above the level at 

 which the temperature began to fall, so that this distribution 

 does not seem to depend on temperature. The number of nau- 

 plii rose to a maximum rate of more than 300,000 per cubic 

 meter in and above the thermocline, but ended with very great 

 abruptness. This termination of the population often took place 

 within the space of half a meter. 



The number of algae also declines very rapidly at the thermo- 

 cline and those which are obtained below this level are dead or 

 dying. The amount of algae thus obtained is, however, far 

 greater than the number of Crustacea; indeed the algae below 

 the thermocline are many times more abundant in rela- 

 tion to the number of Crustacea present than is the case in lakes 

 like those of the Oconomowoc system, in which there is a large 

 crustacean population in the lower waters. It is obvious, there- 

 fore, that the exclusion of the Crustacea from these deeper 

 waters is not due to the absence of food. 



The algae at times appear to accumulate above the thermo- 

 cline, and to pass it, as they settle, only after considerable 

 delay. I have attempted to discover whether this delay was 

 due to the greater density of the water, occasioned by the dim- 

 inution in temperature. A large glass tube, six centimeters 

 in internal diameter and about two meters long, was filled with 

 water and the lower half meter placed in a vessel of ice-water. 

 After a few hours a very marked thermocline was formed, the 

 temperature falling some 6° C. in the space of about 10 cm. 

 Water containing algae, chiefly diatoms, was introduced at the 

 top of the tube and the algae gradually sank through the water. 

 On reaching the artificial thermocline they paused for a few 

 minutes, but rapidly acquired the temperature of the water, as 

 would be expected, and then sank to the bottom of the vessel. 

 The delay at the thermocline could not have amounted to more 

 five minutes for an individual alga. It seems probable from 

 27 



