The Distribution at the Thermocline. 415* 



Cladocera aggregate at the surface at night, but find that the 

 upper water, in the early part of the night at any rate, is ten- 

 anted by a larger proportion of Copepoda than of Cladocera and 

 that a smaller fraction of adult Cladocera is found among those 

 present at this level than at the depth of half a meter, or more. 

 I do not find that a strong wind brings about an even distribu- 

 tion of the Crustacea, although it assists in doing so. In 

 moderate winds the Crustacea approach somewhat nearer the sur- 

 face than in quiet, sunny weather, and during violent winds the- 

 distribution in the upper three meters is more uniform than in 

 cloudy weather, but in case large numbers of young are present, 

 there is always a high percentage in the upper meter. 



THE DISTRIBUTION AT THE THERMOCLINE. 



During the latter part of the summer of 1896 observations 

 were made with the net, in order to determine more exactly the 

 distribution of the Crustacea at the thermocline. The net was 

 raised from the bottom of the lake to the bottom of the ther- 

 mocline and then closed and drawn to the surface. After wash- 

 ing out the collection it was lowered to the depth at which it 

 was closed, opened, raised through one meter and closed again. 

 In this way the population was determined by single meters for 

 the two or more meters including the thermocline and the 

 water immediately above. Great care was taken that the move- 

 ment of the net should be regular, and the messenger was sent 

 down the line in such a way as to close the net immediately on 

 its reaching the upper level of the meter under investigation. 

 The results show that the crustacean population usually passes 

 into the thermocline and often toward its lower part, but that here 

 it ends often with great abruptness. If the temperature conditions 

 are such that the thermocline is spread out over two or three 

 meters the population ends less abruptly than when the thermo- 

 cline is concentrated into a meter or a half meter. The obser- 

 vations showed a population per cubic meter of only a few hun- 

 dred below the thermocline, while in it and above it the popu- 

 lation might range from 40,000 to 60,000 per cubic meter. As 

 these observations agree in general with the more exact results 

 reached by the pump in 1897, the details will not be given. 



