Scott: Lakes of Tippecanoe Basin 



17 



by shallow water. Observations were taken in two of these on 

 the same afternoon. These two basins have approximately the 

 same depth. The large one, in the open exposed part of the lake, 

 had oxygen in small quantity on the bottom and a temperature 

 of 54°, while the smaller protected kettle hole near the west 

 side had a temperature of 45.5° and no oxygen at the bottom. 

 It is perfectly evident from these data that the latter region of 

 the lake was stratified much sooner than the former. 



These lakes that have an oxygenless region duiing summer 

 have also extensive growths of attached aquatic plants in the 

 littoral region. In the latter part of August these become covered 

 with blue-green algae (chiefly Oscillatoria). Many of them die, 

 break from the substratum, and become united into mats at the 

 surface. These are blown about by the wind. I have seen mats 

 a square meter in area near the center of Eagle lake. Some of this 

 material is washed ashore, but with the cooling of the water and 

 consequent death of the blue-green algae, much of this material 

 sinks. 



This is the source of a large part of organic substance that 

 depletes the oxygen at the bottom of the lake. Juday (1911) 

 finds in many Wisconsin lakes that the organic material at the 

 bottom of lakes is derived from the leaves of trees. I have so 

 far found no Indiana lakes in which this factor seemed to be 

 potent. 



Carbon Dioxide 



The relation of the carbon dioxide and carbonates to photo- 

 synthesis and consequently the productivity of waters is well 

 known. One of the clearest statements of these relations is made 

 by Juday (1911). Briefly these relations are the following: 

 Carbon dioxide exists in a lake in three forms, first as free carbon 

 dioxide, CO2 in solution; second, as the acid radical of normal 

 carbonates, chiefly of calcium (CaCOs) or magnesium (Mg. CO3); 

 third, as the second member of the bicarbonates formula of the 

 same metals. In the first and third of these forms it is available 

 for photosynthesis. Only a relatively small amount of free CO 2 

 can be dissolved in water under ordinary conditions. However, 

 when the first two of these, namely, free CO 2, and normal car- 

 bonates, are present in solution, the third is formed. Thus the 

 presence of normal carbonates makes it possible for lakes, during 

 the periods of complete circulation, to store available CO2 by the 

 formation of bicarbonates. 



