72 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



will be most rapid at the end of the flood tide, since at this time the vertical 

 distribution of salinity and density is most uniform. Any increase in wind 

 force will increase the rate of this mixing, which will tend to diminish the 

 salinity gradients still further. 



The distribution of bacteria in relation to the phase of the tide in 

 Mount Hope Bay is shown in Figure 7. In this figure the bacteria are 

 represented by a ratio between the number actually found and the number 

 expected. The number expected is calculated by correcting the number 

 introduced in the river water at the head of the bay by the degree to 

 which this water has been diluted by mixing with sea water. The ratio 

 thus obtained is plotted against the time after high water at each of the 

 locations shown in this figure. If dilution were the only factor operative 

 in this area all of these curves would be flat and show no variation with 

 the tide. The fact that all of them increase above unity indicates that 

 there are contributions of bacteria from sources other than the river mouth. 

 This is to be expected since the area is densely populated and several 

 sewerage systems empty directly into the bay. The interesting fact, how- 

 ever, is that in spite of this additional pollution, the numbers at each loca- 

 tion fall far below the expected numbers during the period of high tide. 

 Since the effect of dilution has been cancelled out it is clear that this 

 diminution is the result of other processes which deplete the bacterial 

 population. 



To summarize, our investigations have indicated that the coliform 

 bacteria disappear rapidly from normal sea water under laboratory condi- 

 tions. This disappearance is not related in a simple way to the chemical 

 content of sea water since autoclaving the water eliminates its bactericidal 

 activity. The lethal factors, or substances, are apparently organic in 

 nature and heat labile. 



That the bactericidal property of sea water is important under natural 

 conditions is indicated by the fact that the disappearance of coliform bac- 

 teria in the sea is much more rapid than can be accounted for by the 

 dilution of contaminated water with sea water. It is fortunate indeed that 

 this is the case, else our heavily polluted harbors would be unbearable. 



Several additional problems must be studied in order to complete the 

 picture. It would, of course, be desirable to determine the nature of the 

 bactericidal substance or factor in sea water. The distribution of bac- 

 tericidal activity in waters and muds collected from polluted areas and 

 at varying distances from shore should aid in identifying its character. 

 The potency of a given body of water may be expected to have an im- 

 portant relation to its ability to accommodate introduced pollution. Finally, 

 considerably more fundamental information is needed concerning the 

 principles which govern the mixing of water masses. When this informa- 

 tion is available it should be possible to plan marine outfalls so that the 

 introduction of pollution will lead to the minimum interference with the 

 fisheries and economy of the area. 



