Huff : RESPONSE OF MICRO-ORGANISMS TO COPPER SULPHATE 419 



numbers, and by December 11, about the time when the lake became 

 frozen over for the winter, this organism had been reduced to about 

 4,920 Standard units per cubic centimter, and was still going down 

 very rapidly. 



The response of four other diatoms to the copper sulphate treat- 

 ments is shown in figure 2. These four, Melosira grannlata, Cyclo- 

 tella conita, Asterionella formosa, and Fragilaria capiicina, were the 

 only diatoms, aside frorii the two mentioned above, that occurred 

 commonly in the main body of the lake. The first copper treatment 

 resulted in the complete elimination of Cyclotella comta for the sea- 

 son. Melosira^ which was greatly reduced by the first treatment, 

 was practically eliminated by the second, not to occur in large num- 

 bers again until late in September, after the autumnal circulation had 

 begun. Asterionella and Fragilaria were greatly reduced by the first 

 treatment, and practically eliminated by the second treatment. Both 

 of these had made a fair start, however, late in August, but the third 

 treatment cut them down at this time and not until the autumnal 

 circulation of late September and October, did they re-establish 

 themselves in even fairly large numbers. These, like Stephanodiscus, 

 were greatly reduced as soon as the autumnal circulation had ceased, 

 and before the lake had started to freeze over. 



The several species of green algae common in Vadnais Lake vary 

 a good deal in their sensitiveness to copper sulphate. Spirogyra. 

 for example, which was quite common, from 500 to 600 standard 

 units per cubic centimeter at the time of the first treatment, was en- 

 tirely eliminated within three or four days after the treatment. Just 

 before the second treatment it occurred again in small quantities, 

 there being from 100 to 150 standard units per cubic centimeter. 

 With the second treatment it disappeared completely and not even 

 a trace of it was found in the main body of the lake the remainder 

 of the summer. Some other forms, however, especially Eudorina 

 and Pandorina were found to be, at tijnes, more resistant. These 

 two forms, as is shown in figure 3, were running from 300 to 400 

 standard units per cubic centimeter at the time of the first treatment. 

 They showed a decided decrease in numbers shortly after the treat- 

 ment, but continued to oscillate a good deal between 50 and 300 

 standard units per cubic centimeter until after the second treatment, 

 July 12. With this treatment both of these forms and two others, 

 that had occurred in smaller numbers, namely Pediastrum and 

 Staiirasirum, were practically eliminated, none of them running 



