THE RESPIRATION OF AN INLAND LAKE 339 



inland lakes, even those whose depth is two hundred feet or more, 

 become almost, or quite, saturated with oxygen at a temperature but 

 little above the freezing point. This quantity amounts to about 10 c.c. 

 per liter, or about 1 per cent, by volume; nearly twice as much as the 

 water will hold at the highest summer temperature. In this condi- 

 tion as regards oxygen the lake goes into winter quarters, becomes 

 covered with a sheet of ice in our latitudes, and is, therefore, shut off 

 until spring from all further direct connection with the atmosphere. 

 During this period the stock of oxygen is used up to some extent, espe- 

 cially in the water adjacent to the bottom. But as the vital processes 

 of both plants and animals, and also those connected with decay, go on 

 slowly at the low temperature of the water in winter, the amount of 

 oxygen thus consumed is comparatively small, and most lakes contain 

 an abundance for all forms of life at all depths, except perhaps in the 

 strata very close to the bottom. This statement, though generally true, 

 will not hold universally. In some ponds which are shallow and con- 

 tain a large amount both of living organisms and of decomposing mat- 

 ter, the oxygen beneath the ice may become wholly used up. We all 

 know of lakes where, if a hole is cut through the ice in late winter, 

 the fish will crowd to it for air so eagerly and in such numbers as to 

 be forced out on the ice. There are on record cases where an unusual 

 exhaustion of the oxygen below the ice of a lake has caused the death 

 of most of the fish. Such cases, however, are not common, and in the 

 great majority of lakes the consumption of oxygen in winter does not 

 go far enough to affect unfavorably their living inhabitants. 



Associated with this partial exhaustion of oxygen, there is an in- 

 crease during winter of the amount of carbon dioxide — the main gaseous 

 product of respiration. This is not present in any observable quantity 

 in the lake at the time of freezing, but it increases during the winter 

 and the quantity at the bottom may become very considerable. The 

 amount will be, in general, proportional to the amount of oxygen used 

 up. In the spring, when the ice has melted, the water of the lake is 

 once more uniform in temperature. It is put into motion once more 

 by the wind and all parts of the water are brought into contact with the 

 air. The carbon dioxide, which has been accumulating during the 

 winter, is discharged or used by plants and the lake again becomes 

 nearly saturated with oxygen. But, as the temperature in spring is 

 higher than in the autumn, the amount of oxygen taken in is less, and 

 since the temperature of the water continues to rise, the stock of oxygen 

 is being diminished from this cause quite independently of any use 

 made of the gas by the organisms of the lake. 



The period of full oxygen saturation in the spring is a brief one in 

 our climate. The season advances very rapidly and the surface water 

 soon acquires a higher temperature than that at the bottom. This 



