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 tho 
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. 
