226 
Throughout the entire period of strati- 
fication there was at least 0.5 degree dit- 
ference between temperatures at 15- and 
18-foot depths and at least 1 degree dif- 
ference between temperatures at 12- and 
15-foot depths. Usually the differences 
were | to 3 degrees between the 15- and 
18-foot depths and 1.5 to + degrees be- 
tween the 12- and 15-foot depths. 
The first period of rapid increase of 
water temperatures at the 15- and 18-foot 
depths occurred in early May following 
heavy rains of May 3 and 6. At the be- 
ginning of this period, the temperatures 
at the 15- and 18-foot depths were 51.0 
and 50.0 degrees F., respectively. By early 
June, the temperatures at the 15- and 18- 
foot depths had risen to 64.0 and 63.5 
degrees F., respectively. By June 7, when 
rains were resumed after a period of very 
low precipitation, temperatures at the 15- 
and 18-foot depths had dropped to 61.0 
and 58.0 degrees F., respectively. Heavy 
rains occurred at rather frequent inter- 
vals from June 7 to July 14, and during 
this period the temperature of water at 
the 15-foot depth rose from 61.0 to 72.0 
degrees F. and at the 18-foot depth from 
58.0 to 70.5 degrees F. During this same 
period, the temperature of the surface 
water dropped from a high of 87.0 de- 
grees F. on June 2 to 82.5 degrees F. on 
July 14. 
The fact that periods of rapid tempera- 
ture rise in the water close to the lake 
bottom corresponded to periods of heavy 
rainfall indicated that the loss of the cold 
bottom water through the spillway tower 
was largely responsible for these tempera- 
ture increases. 
Fluctuations in the dissolved oxygen 
content of the deeper waters of Ridge 
Lake were even more striking than those 
in temperature. Following the beginning 
of prolonged stratification in April, 1942, 
the supply of dissolved oxygen at the 15- 
and 18-foot depths gradually became less, 
except when, during the second week in 
May, two rains, each exceeding 1 inch of 
precipitation, fig. 5, apparently forced 
out of the lake a large volume of the 
bottom water containing a reduced supply 
of dissolved oxygen. This water was re- 
placed by that lying nearer the surface 
and containing a greater amount of oxy- 
gen. ‘Thus, there was a temporary in- 
Ittrnois NaturAL History Survey BULLETIN 
Vol. 26, Art. 2 
crease of oxygen at the 15- and 18-foot 
depths; but, because of the oxygen demand 
of organic material on and above the lake 
bottom, the new supply rapidly was used 
up until by early June no dissolved oxy- 
gen remained. On June 7, a period of 
heavy rainfall began, fig. 5, which ended 
July 14; there were 7 consecutive days 
(last + days in June and first 3 days in 
July) when no rain fell. The heavy rains 
of this period forced the oxygen-deficient 
waters out of the lake and almost imme- 
diately increased the available dissolved 
oxygen to more than + parts per million 
at the 15-foot depth and considerably 
more at the 12-foot depth, fig. 6. Samples 
for oxygen analyses taken at these depths 
and at the 18-foot depth (this sample was 
taken 8 inches off the bottom) on July 11 
showed that the dissolved oxygen was 6.3 
p.p.m. at 12 feet, 5.1 p.p.m. at 15 feet, 
and 2.7 p.p.m. at 18 feet. Before the end of 
the third week in July, the dissolved oxy- 
gen at 15- and 18-foot depths had again 
dropped to zero, where it remained until 
the fall overturn began on September 27, 
as a result of abnormally cold weather dur- 
ing the preceding week. At the time the 
fall overturn began, water temperature 
at a depth of 4 feet was 62.0 degrees F. 
The biological effect of the removal of 
stagnant water from the lower levels of 
Ridge Lake is unknown. Certainly it 
must temporarily increase the volume of 
water inhabitable by fish. Evidence from — 
oxygen determinations made in 1948 by — 
Fleming (1949: 20) and by other workers | 
in 1942 and 1943 shows that, after periods 
of flushing, the increased oxygen supply 
is rather rapidly used up, probably too 
soon for the invasion of those bottom or- 
ganisms that require dissolved oxygen. 
The increase in water temperatures may 
speed up the growth of midge larvae 
(Chaoborus) that normally inhabit oxy- 
gen-deficient bottoms. It is probable that 
the perpendicular extent of the oxygen- 
deficient stratum of water above the bot- 
tom is, on the average, somewhat less than 
it would be without the tower spillway 
arrangement for loss of bottom water. 
Fertility of Water.—Ridge Lake 
probably ranks intermediate in fertility 
when compared with other lakes within 
the boundaries of Illinois. Judged by fish 
production, most ponds and lakes in central 
a 
