158 Thirty-fifth Annual Meeting 
below this depth it rapidly declines until a little below 9 meters 
no oxygen is left. This lake gets from the surface water and 
from the shore a great amount of decomposable material and 
one might expect that the oxygen would be very promptly ex- 
hausted up to the top of the cool water, especially as the warm 
upper stratum is formed early in May or even late in April. 
The diagram, however, shows that the facts are very different 
and that the upper part of the cool water, so far from being 
poorer in oxygen than the upper stratum, has a much larger 
amount. The cause of this great amount of oxygen is as fol- 
lows: The transparency of the water is such that the algae of 
the water can grow at a depth considerably greater than the top 
of the cold water; and these algae, lying as they’ do im the water 
which is not distributed by circulation, the cool water lying be- 
low the warmer stratum, get light enough to utilize as food the 
carbon dioxide and the other products of decomposition that 
come to them, and they liberate free oxygen as a result of that 
process. Thus we get in the upper part of the cool water not 
merely the normal supply but an over-saturation of oxygen, an 
amount which could not be held in the water at all if that water 
were at the surface. In this way the thickness of the laver of 
water which is available for animal life is practically doubled 
by the presence of the oxygen which is manufactured by the 
plants. 
The diagram of Elkhart Lake, Fig. 11, shows the same thing. 
This is a lake about 110 acres in area, and 112 feet in depth, 34 
meters. The upper stratum, the layer of warm water, is about 6 
meters in thickness, and the temperature falls off very rapidly 
from that depth. At 6 meters the oxygen begins to show an in- 
crease ; at 8 meters a maximum of oxygen is reached amounting 
to more than the 11 cc. per liter. It does not begin to fall off 
very greatly until 10 meters have been reached, and even at 12 
meters there is still a somewhat abundant supply. From that 
point it declines until it practically reaches zero, although it does 

at least not 
in August, so far as our observations go. So that in this lake also 
not get absolutely to zero at any point in this lake 
the stratum which js available for animal life is by this action 
of the plants increased from a thickness of perhaps 6 meters to 
12 or more meters. 
