Huff: RESPONSE OF MICRO-ORGANISMS TO COPPER SULPHATE 423 



times in Vadnais Lake is to be sought in the conditions existing in 

 this lake favorable to their growth and reproduction, rather than to 

 any large number that may be found pouring into the lake at any 

 one time through the inlet. 



Of the four groups of organisms entering the lake, the Cyano- 

 pliyceac, or blue-green algae, are usually most numerous, and appear 

 to be, as a group, most sensitive to the copper sulphate. For about 

 ten weeks during the summer, they ran from 1,000 to 7,000 standard 

 units per cubic centimeter at the inlet, with an average for this period 

 of about of about 2,500 standard units per cubic centimeter. At the 

 outlet of the lake they w^ere running about 2,000 standard units 

 per cubic centimeter at the beginning of this period, but after the 

 first treatment they rapidly disappeared and during the last six 

 weeks of this period scarcely a trace of them was found. On Au- 

 gust 27, at the time of the third treatment of Vadnais Lake, the creek 

 entering the lake, and Sucker Lake, a small lake a short distance 

 above Vadnais, were treated, and for the first time during the sum- 

 mer the organisms practically disappeared from the waters of the 

 inlet. Although the treatment here was considerably heavier than 

 in Vadnais Lake, the effects w^ere of shorter duration, for the 

 smaller size of Sucker Lake and the creek permits a more rapid dis- 

 placement of the treated water by untreated water from above. 



From the tenth of June to the twenty-seventh of August, the 

 average number of organisms in the inlet of Vadnais Lake was more 

 than 4,000 standard units per cubic centimeter. Compare these fig- 

 ures with the number found in the samples taken at the weir below 

 the gatehouse, especially for a period of about five or six weeks fol- 

 lowing the copper sulphate treatments of the lake, when the number 

 here was below 100 standard units per cubic centimeter, and the 

 effectiveness of a treatment of 1 to 10,000,000 can not be questioned. 



While many organisms appeared at one time or another during 

 the summer in the waters of Vadnais Lake, and disappeared, prob- 

 ably as a result of the copper treatment, many of them were not 

 found in sufficiently large numbers to justify a statement concerning 

 the exact effect of copper sulphate upon their existence. With 

 several others, however, that were more common, there can be no 

 question as to the effect of copper sulphate. The following forms 

 were found before one or more of the treatments, in sufficient num- 

 bers to justify certain conclusions concerning the effects of the 

 treatment as here given. 



