388 
BULLETIN OF BUREAU OF FISHERIES 
neutralizing actions of these fixed carbonates on free acids. Again, the fixed carbon 
dioxide is of considerable importance in detoxifying various effluents, particularly 
those carrying salts of heavy metals, by precipitating various compounds, since many 
carbonates are relatively insoluble and, therefore, are removed at least temporarily 
from the waters during precipitation. This is discussed more fully in the section on 
metallic poisons. 
J. MISSOURI HEADWATERS K.OHIO L. TENNESSEE 
16 STATIONS 33 STATIONS 103 STATIONS 
121 DETERMINATIONS IGS DETERMINATIONS 163 DETERMINATIONS 
M. GULF COAST STATES 
29 STATIONS 
1,317 DETERMINATIONS 
N. RIO GRANDE 
ft STATIONS 
129 DETERMINATIONS 
O. COLORADO RIVER SYSTEM 
10 STATIONS 
Ml DETERMINATIONS 
100 , — 
GO 
©Oh 
P. COLUMBIA RIVER SYSTEM 
33 STATIONS 
265 DETERMINATIONS 
IOO, 
Figure 17. — Continuation of comparisons of fixed carbon dioxide values, stippled graph A figure 16 being the standard. 
FREE CARBON DIOXIDE 
Figures 18 and 19 present the data from 3,351 determinations of free carbon 
dioxide in various streams of United States. From these figures, and particularly from 
the graph of the composite, it may be seen that river and stream waters, where good 
fish faunae were foimd, carried consistantly less than 5 cubic centimeters of free 
carbon dioxide per liter, 90 percent of the waters where good fish faunae were taken 
carrying less than 2 cubic centimeters of free carbon dioxide per liter. It must be 
pointed out in this connection that conditions in moving streams are quite different 
as regards free carbon dioxide from those obtaining in lakes and ponds. Due to the 
constant turnover of river and stream waters because of current action, there is a very 
uniform mixing of water from surface to bottom (see data from Grand Tower on the 
Mississippi and Pan Eddy in the Tennessee (table l)), so that the free carbon dioxide, 
as well as the other dissolved gases, is quite uniformly distributed throughout the 
