552 
BULLETIN OE THE BUREAU OF FISHERIES. 
word was proposed by the senior author of this paper in 1897 as an equivalent for Richter’s 
term “sprungschicht,” and was so defined. In this sense it has been included in the New 
Oxford Dictionary and also in the Century Dictionary. In 1912 Wesenberg-Lund ® 
redefined the word so as to restrict it to the meter of maximum fall of temperature, 
retaining the word “sprungschicht” for the larger stratum. We see no sufficient reason 
for this change, which would force a writer in English to invent a new equivalent for 
“sprungschicht” or else employ some long paraphrase for that term, such as Wedder- 
bum’s “discontinuity layer.” We therefore retain the term “ thermocline ” in its 
original sense, in which it has been adopted by English dictionaries. 
The terms “epilimnion,” “thermocline,” and “ hypolimnion ” are derived from 
that thermal condition of the lake which extends from early midsummer to the beginning 
of the homothermal period in autumn. Like all conditions that arise as a result of 
growth, this one comes on gradually, and its beginnings are not easy to define. In this 
paper the situation is treated and discussed as it appears in late summer during the 
equithermal period. Table ii shows that the thermocline in the six major lakes (or 
seven with the addition of Green Lake, Wis.) was from 5 to 9 meters thick; that the 
fall of temperature in it was from 7.3° to 12.3°; and that this fall represented from 
about 40 per cent to nearly 80 per cent of the difference of temperature between the 
surface and the bottom of the lake. In the smaller lakes it is 4 to 6 meters thick, but 
contains a decline of temperature nearly as great as that of the larger lakes and one 
which represents a higher average percentage of the total fall. 
These figures have little significance in their details; since, as already stated, the 
thermocline is subject to constant alterations of thickness due to the oscillations of the 
water of the lakes. These changes may continually and rapidly alter the average rate 
of fall of temperature, and the total number of degrees included in the thermocline; the 
position of the meter of maximum descent of temperature, and the amount of fall 
included in it. The figures therefore represent in their range of variation about what 
might be expected in lakes of this size under average conditions of summer weather. 
Any of the major lakes might, under suitable conditions, show a thermocline like that 
of any one on the list. 
The general result, however, shows more than the single observation. The thermo- 
cline lies deeper in the larger lakes than in the smaller, and on the average is over 1.5 
meters thicker. This region represents the stratum in which the effects of the direct 
wind circulation die out, just as the epilimnion is the stratum in which a direct wind 
circulation is made possible by the cooling effect of night and of cool periods. It might 
be thought that in the larger lakes the greater influence of the winds would make the 
descent of temperature in the thermocline more gradual. This is true to a limited 
extent, as is best seen in the thermocline of Seneca Lake. It is more evident when large 
lakes are compared with very small ones. In general, however, the greater effect of 
wind in the larger lakes is rather to increase the thickness of the epilimnion than to 
® Bronsted, J. N., and Wesenburg-Lund, C.: Chemisch-physikalische Untersuchungen der danischen Gewasser. Inter- 
nationale Revue der gesamten Hydrobiologie und Hydrographic, bd. iv, 1911, Biologisches Supplement, sr. n, p. 262. Leipzig, 
1911-12. 
