422 



MINNKSOTA nOTAMCAL Sri"l)Ii:S 



shown in figure 4. Running from about 200 to 300 standard units 

 per cubic centimeter at the time of the first treatment, they disap- 

 peared for about ten days. They reappeared in similar numbers 

 before the second treatment, but after the second treatment disap- 

 peared rather suddenly and only slight traces of them occurred again 

 durins: the entire season. 



Fig. 5. Relative numbers of microorganisms entering Vadnais Lake through waters of 

 the inlet. 



In order to get a Ijetter idea of the effect of cop])er sulphate upon 

 the various organisms found in Vadnais Lake it may be well to com- 

 pare the numbers of some groups entering the lake at the inlet (fig- 

 ure 5), with those in the water leaving the lake as indicated by 

 figures 1 to 4. The two most abundant forms in Vadnais Lake, 

 namely Synedra pulchella and Stephanodiscus niagarae, are forms 

 that prefer the deep water, and while they enter the lake in small 

 numbers, the explanation for the very large number occurring at 



