Page:Popular Science Monthly Volume 72.djvu/351

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which is taking place rapidly in the lower water; and not infrequently a far larger amount of organic life may be found in these strata than in any other portion of the lake. This process is necessarily limited

PSM V72 D351 Elkhart lake chart of water depth and oxygen level.png

Fig. 1. Elkhart Lake, Aug. 23, 1905. The vertical spaces indicate the depth in meters. The horizontal spares indicate cubic centimeters of oxygen per liter (parts per thousand of volume), as shown by the line O---O. They also indicate the temperature in centigrade degrees, as shown by the line T-—T. The upper, warm layer of water is about 6 m. thick; the thermocline extends from 6 m. to about 10 m. The presence of the manufactured oxygen is very plainly seen in the space between 5 m. and 12 m.; the amount of oxygen rising to a maximum of over 10 c.c. per liter at the depth of 8 m. The relation of this gas to the temperature of the water and the consequent stratification is clearly shown.

to lakes whose upper warm layer is thin, and is confined to the upper part of the cold water, since only there can the light have sufficient intensity to carry on the operation. But even thus restricted, it is of great value to some lakes.

I have said little hitherto of the carbon dioxide—a gas whose importance is quite equal to that of oxygen—and now can only sketch a part of its complex story. This gas plays many roles in the respiration of the lake. It is at once the waste product of the tissue activity of plant and animal, the product or by-product of decomposition, and the indispensable food of green plants. The lake may obtain the gas from the air, and to some extent does so. Carbon dioxide exists in the atmosphere in very small amount—about four parts in 10,000. Minute as this quantity is, the land plants are able to secure from it ample supplies of carbon. The movement of the air is so free and such enormous quantities pass over the surface of the plants, that they readily pick up the gas in large amounts. But the situation of the algæ and other plants of the lake is very different, as they must secure their