414 MINNESOTA BOTANICAL STUDIES 



second treatment, the organisms, especially Synedra, began to show 

 great increase in numbers. In seven days the total number of or- 

 ganisms increased from 100 standard units per cubic centimeter, to 

 3,952 standard units per cubic centimeter. A third treatment of 

 copper sulphate on August 27, of the same strength as the second 

 (1 to 10,000,000) given more than a month before, put a sudden 

 check upon the increase, and brought them down, a little more 

 slowly than before, but by the fourteenth of September they were 

 again below 100 standard units per cubic centimeter and remained 

 comparatively low for about another month, many of them not in- 

 creasing again during the entire season. 



During the summer months the water in the bottom of the lake 

 has a temperature of from 10 to 20 degrees Fahrenheit lower than 

 that of the surface. The maximum temperature for the bottom of 

 the lake was about 59 degrees Fahrenheit, which was reached late 

 in July. The surface temperature was sometimes as high as 78 

 degrees Fahrenheit. The colder, heavier water of the greater depth 

 is not mixed with the lighter water above by ordinary surface dis- 

 turbances, and therefore becomes quite stagnant during the several 

 months when the surface water is comparatively warm. During 

 September, however, with the short days and cooler nights, the 

 water of the surface of the lake begins to cool and continues grad- 

 ually until the lake freezes over in December. The water cools, 

 of course, from the surface, and whenever the surface becomes 

 cooler than the water of the bottom, this cooler, heavier surface 

 water settles to the bottom, and the warmer water of the 

 bottom rises to the surface. This circulation, or overturning proc- 

 ess, will continue for several weeks until the temperature of all of 

 the water in the lake has lowered to 39.2 degrees Fahrenheit, where 

 it reaches its maximum density. In Vadnais Lake in 1915, this 

 overturning process began about September 24, and continued until 

 about November 25. In the early stages of this circulation, some 

 organisms, especially diatoms, were more abundant at the bottom, 

 fifty feet below the surface, than they were at the surface. During 

 the greater part of the circulation period, however, the organisms 

 were distributed nearly uniformly from surface to bottom of the 

 lake. This was very different from their distribution during the 

 summer months when the water near the bottom of the lake was 



