154 Thirty-fifth Annual Meeting 
early July; so the bottom water is cut off from access to external 
air for very different lengths of time in different lakes. 
And still further: The amount of the oxygen in the lower 
water depends not merely on the length of time that the bottom 
water is cut off from the external air, but it depends also upon 
the amount of decomposable material discharged into it by the 
upper water and on the volume of lower water, which depends on 
the depth of the lake. If the amount of life in the lake is small, 
the amount of material which will decompose in the bottom 
water is extremely small, and the exhaustion of the oxygen goes 
on with corresponding slowness. If the volume of the lake is 
great, as in Green Lake (237 feet deep) the amount of oxygen is 
correspondingly great, and it is not rapidly used up. [ff the 
lake (like Mendota) has an enormous amount of plant life in the - 
upper water, so that there is a continual and rather rapid rain 
of organic matter dropping down in the lower water, decomposi- 
tion goes on rapidly, aided also by the comparatively high tem- 
perature of the bottom water, and the oxygen is exhausted at a 
comparatively rapid rate. 
Then again, in case of the smaller lakes, the amount of dis- 
composable matter coming in from the margin of the lakes in- 
creases proportionately—the smaller the lakes the larger the 
margin with reference to the volume of the water of the lake. 
There is a zone around the edge of any lake in which the bot- 
tom plants will grow. ‘This zone does not differ in breadth in 
proportion to the size of the lake, so that in a small lake the 
central part which is free from bottom growth is much smaller 
proportionately than in the larger lake, and the material washed 
into the deeper water from the margin and the banks is corres- 
pondingly greater in the smaller lake. Then, too, the leaves 
which are blown into lakes of 20 or 30 acres in area, or even 
larger, form a very important addition to the decomposable ma- 
terial on the bottom. The result of all this, in those small 
ponds and lakes so common in the kettle moraine of Wisconsin, 
is that the bottom water is cut off from access to oxygen at an 
early period in the spring, and a great amount of decomposable 
material of all kinds is present in the lake. It follows that there 
is a long period during which there is no oxygen in the lower 
water, and consequently no animal hfe, and the bottom of the 
