1278 



BULLETIN OP THE BUREAU OF FISHERIES. 



temperature is nearly the same at all depths, rising from less than one degree 

 just below the ice to something over one degree at the bottom. The water con- 

 tains about 9 cubic centimeters of oxygen per liter, except in the bottom, where 

 it has begun to disappear under the action of decomposition. Nitrogen is present 

 to about the amount required to saturate water at the given temperature. The 

 reaction of the water is neutral or slightly alkaline at all depths. Carbonates 



are present to an amount rep 

 



n r\ ^ J^ c. C 



CT 



N 34 36 38 40 4? 44 



resented by about 38 cubic 

 centimeters of carbon dioxide 

 per liter. 



As the winter advances (fig. 

 3) some changes occur under 

 the ice . The temperature rises 

 slowly at all depths, but most 

 rapidly at the bottom. Slow 

 decomposition goes on in the 

 deeper water, where also the 

 greater part of the fish are 

 found. There thus results a 

 reduction of the amount of 

 oxygen, which may nearly or 

 quite disappear at the bottom, 

 and a corresponding develop- 

 ment of carbon dioxide, so 

 that as winter advances the 

 bottom water may contain 

 considerable quantities of free 

 carbon dioxide. The reaction 

 of the upper water becomes 

 much more markedly alkaline 

 than in the early winter, a 

 change probably due to the in- 

 fluence of the growing plants. The carbonates may or may not decrease in the 

 water immediately below the ice. If a diminution is found (and such decrease 

 may be very pronounced, as in fig. 4), the change is due to the accumulation 

 beneath the ice of water resulting from the melting of the ice or snow and 

 containing, therefore, less dissolved matter than the water of the lake usually 

 holds. As the season advances a rapid growth of algae may take place beneath 

 the ice, resulting in a considerable increase of the oxygen, which may carry 

 it beyond the point of saturation. This increase is usually accompanied by a 

 considerable increase in the alkalinity of the water (fig. 4) . 



I 

 2 



3 

 4. 

 5 

 6 



I'' 

 6 



9 



10 

 II 



12 



I 



13 

 14 



r 



15 

 16 

 17 



Id 

 19 

 20 

 21 

 22 

 23 



OTC 



Fig. 3.- 



N 



-Lake Mendota, February 25. 1906. 



Cb 



