THE RESPIRATION OF AN INLAND LAKE 349 



which plants feed, both gaseous and other. These are for the time 

 locked up in the lower water and so withdrawn from the circulation 

 of life. In the autumn, as the lake cools and the thickness of the 

 circulating stratum increases, these matters become available so far 

 as they lie in the upper part of the cooler water, and when the lake has 

 become uniform in temperature to the bottom, and the water is turned 

 over by wind, the whole of this accumulated stock is available for the 

 purposes of plant growth. This may be one of the reasons for the 

 abundant growth of alga?, which takes place in the autumn. But while 

 the non-gaseous products of decomposition may be wholly utilized in 

 the lake, the carbon dioxide is hardly likely to find full use. When it 

 once becomes distributed through the water and new portions of the 

 water are being continually exposed to the air, considerable quantities 

 must escape during the hours when plants are unable to avail them- 

 selves of it. 



Thus the rudimentary character of the circulatory apparatus of the 

 lake forms an insuperable obstacle to the best utilization of the food 

 supply. It is therefore easy to see why life is relatively so abundant 

 in large and shallow lakes, in which the circulating methods have a 

 maximum efficiency. The fact that these lakes are shallow permits a 

 larger growth of life, since not only is the water available but plants 

 in large quantities may grow from the bottom. But of even more 

 importance than this relation is the fact that since the entire mass of 

 water is kept in circulation by the wind, all the products of decom- 

 position are immediately available for use and the life cycles of the 

 plants may go on as rapidly as their rhythm of growth will permit. 

 The carbon dioxide and other products of decomposition, instead of 

 being locked up in the deeper water and set free only during that season 

 which is least favorable for growth, are utilized immediately and are 

 employed over and over again through the warmer season as the cycles 

 of life and death of the individual plants recur. It is plain that lakes 

 whose margin is wide and shallow, though the middle may be deep, 

 must stand next to the shallow lake in efficiency of means of transpor- 

 tation. Much growth takes place in the shallow waters, much decom- 

 position goes on there, and relatively little of the organic matter sinks 

 into the deep water, to be withdrawn from circulation. Least favor- 

 ably situated is the deep and steep-sided lake, whose cold depths are 

 continually swallowing almost all of the products of the summer's 

 growth, and give them back for use, only late in the autumn when the 

 season for active life is passing away. 



Some lakes may find aid from another source in the task of securing 

 carbon dioxide. Most natural waters contain a certain amount of 

 calcium and magnesium salts in solution, and, for the greater part, 

 these exist in the form of bicarbonates. Lakes whose water is hard 



