A UMNOIyOGICAL STUDY OF THE FINGER LAKES. 
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populated with such forms. But in some of the larger and deeper lakes a portion or 
possibly all of the excess may have been due to the presence of residual gas, more espe- 
cially since some of them, such as Owasco and Seneca, had a relatively small number 
of chlorophyllaceous organisms. 
In addition to the processes which tend to keep the volume of oxygen at or above 
saturation in this stratum, there are others which tend to reduce it below that point. 
A certain amount is consumed in the respiration of the various organisms inhabiting 
this region, and another portion, perhaps much larger, is removed by the decomposition 
of organic material. The amount of oxygen actually held in solution by this water then 
is the resultant of the processes which tend to maintain an abundant supply and those 
which tend to exhaust it. If consumption exceeds the new supply, the amount falls 
below the saturation point as in Conesus Lake; but if the factors which tend to maintain 
saturation and to raise the amount above this point predominate, the opposite result 
is produced. 
In the hypolimnion . — During the period of thermal stratification, the hypolimnion 
is cut off from contact with the air by the epilimnion and conditions are not favorable 
for photosynthesis in this stratum. As a result the only source of oxygen during this 
time is the small amount which diffuses down from the upper water. But this gas 
diffuses very slowly through water, its coefficient of diffusion being only 1.62, so that 
only a small and negligible amount is obtained in this way. Thus the supply of dis- 
solved oxygen of the lower water is limited to the amount which it possesses at the close 
of the vernal period of circulation. Any decrease during the summer remains as a 
deficiency until the autumnal overturn takes place and the normal amount is not regained 
until after complete autumnal circulation is established. 
As might be expected from the above conditions, different lakes show wide differences 
with respect to the quantity of dissolved oxygen in the hypolimnion in late summer; 
but they readily fall into three groups. The first includes those lakes in which some 
or practically all of the hypolimnion is devoid of free oxygen in the later stage of the 
stratification period; the second comprises those lakes in which there is a marked 
decrease of dissolved oxygen in some or all of the hypolimnion but not an actual disap- 
pearance of it; and the third class includes those lakes in which the decrease of the 
oxygen in this stratum is comparatively slight. These differences in the dissolved 
oxygen content depend, in the main, upon two factors, viz, the amount of decomposition 
taking place in the hypolimnion (which may be characterized as the zone of decompo- 
sition) and upon the volume of this stratum. If the epilimnion contributes a large 
amount of decomposable material in the form of dead plankton organisms to the lower 
water and the volume of the hypolimnion is relatively small, most or practically all of 
the dissolved oxygen in the stratum may be consumed before the autumnal overturning. 
Before the oxygen all disappears, some of it is also consumed in the respiration of organ- 
isms which may occupy this region, such for example as fishes and plankton Crustacea. 
But decomposition is a much more important factor in the removal of oxygen from this 
stratum. 
