The Distribution at the Thermocline. 
417 
The distribution of the nauplii at the thermocline is especially 
noteworthy. During the period of the observations there w ere 
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 
