Juday—Lakes in Central America . 
23a 
Unlike the other three lakes the water of lake Ilopango gave 
an acid reaction at all depths. In the upper 25 m. the acidity 
was equivalent to 2.78 cc. of free carbon dioxide per liter of 
water, while in the bottom stratum it amounted to 7.23 cc. Such 
a marked acidity following so soon after the winter period of 
circulation would seem to indicate that the water of this lake at 
all depths may have an acid reaction throughout the year. Sim¬ 
ilar results have been obtained on soft water lakes in Wisconsin, 
except that such a high degree of acidity has not been found in 
the upper water of the latter lakes, their acidity rarely exceeding 
one cubic centimeter per liter of water in the epilimnion. Also 
in temperate lakes which possess as large an amount of fixed car¬ 
bon dioxide as lake Ilopango the water becomes alkaline soon 
after the vernal overturning has taken place and the water of 
the epilimnion, at least, remains more or less alkaline until the 
time of the autumnal overturn. But the bottom water of such 
a lakq may possess a very high degree of acidity toward the 
close of the summer period of stratification. At this time it 
may be equivalent to as much as 30 cc. to 50 cc. of free carbon 
dioxide per liter of water. Doubtless the acidity of the lower 
water in lake Ilopango becomes much greater as the season ad¬ 
vances, and it may equal or exceed that of these temperate lakes 
just before the winter period of circulation is inaugurated. 
In general there are four sources of carbon dioxide for lake 
waters. They are (1) the air, (2) the decomposition of organic 
substances, (3) the respiration of the organisms inhabiting the 
water, and (4) ground waters. The atmosphere contains from 
three to four parts of carbon dioxide per 10,000 so that water 
which is freely exposed to the air absorbs a small amount of this 
gas. But only a relatively small amount is obtained from this 
source because it is absorbed only in proportion to its partial 
pressure which is slight. Lake waters are generally well popu¬ 
lated with organisms which furnish a supply of decomposable 
material at their death. The decomposition takes place at all 
depths but is most vigorous at the bottom where the organic ma¬ 
terial is most abundant. In their respiration these organisms 
consume dissolved oxygen and liberate carbon dioxide. Many of 
the lower organisms are able to live in water which contains no 
free oxygen, but in the intra-molecular respiration which they 
carry on a certain amount of carbon dioxide is doubtless im¬ 
parted to the water. 
