406 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1939 



A TYPICAL INVASION OF DIATOMS 



The troubles of a water engineer usually occur suddenly and with- 

 out warning. An example of this occurred at the MacMillan Reser- 

 voir in Washington, D. C, in July 1938. During the early part of 

 July there was no unusual evidence of organisms in the water. On 

 July 26, however, there appeared, together with an upset oxygen 

 and carbon dioxide balance in the water, large quantities of the 

 diatom Synedra pulchella (pi. 3, fig. 1), which consists of several 

 cells each 40 microns in length fastened together like a comb. Cop- 

 per sulfate in large amounts was used to kill these diatoms. It was 

 distributed from a skiff with an outboard motor which circled the 

 reservoir and cross-hatched the dead pockets. The response of the 

 organisms to the copper sulfate is shown by the following : 



Pounds 

 Single of copper 



cells sulfate 



Date per cc. added 



July 26 1, 692 



July 27 338 500 



.July 28 352 500 



July 29 1(X) 200 



July 30 58 



July 31 0.9 



Aug. 1 0.0 



Total 1,200 



A study of the bacterial records for the previous month showed that 

 there had been a decided decrease in the bacterial efficiency and that 

 the dissolved oxygen of the filtered water was as low as 50 percent and 

 on some filters even 40 percent, whereas the normal summer-dissolved 

 oxygen is usually never lower than 70 percent. The increased amount 

 of carbon dioxide in the water undoubtedly caused increased diatom 

 production which was already favored by the large silica content of 

 the streams. 



In 1934, when a large outbreak of Synedra delicatissima (pi. 3, fig. 

 2) occurred at Dalecarlia, one part per million of copper sulfate was 

 added to the water for 3 days. The diatoms blocked the filters so 

 completely that it was necessary to wash them every 15 minutes. Al- 

 though the copper sulfate killed the microorganisms, they were not 

 deposited in the settling basins, since they were about the same density 

 as water. 



As a rule, a dosage of 0.2 to 0.5 part of copper sulfate per million 

 is sufficient to kiU the plankton at Dalecarlia. Microorganic analysis 

 is part of the regular routine. Samples of the water are collected from 

 the river daily and nm through the Sedgwick-Eafter filters for 

 examination. Usually, the treatment is necessary only when turbidity 

 is between 15 and 50 parts per million. "Wlien heavy waves of mud 



