STUDIES OF COLIFORM BACTERIA IN THE SEA 



69 



The bacterial numbers are, however, always lower than would be pre- 

 dicted on the basis of dilution alone. To illustrate this point the dilution, 

 expressed as percent outfall water in the sample,* and the surviving per- 

 centage of bacteria in the sections across and along the axis of current 

 flow are plotted in Figure 4. If the bacterial count decreased only because 

 of the dilution of the water the two sets of data would be superimposable. 

 The surviving percentage of bacteria, however, is always less than the 

 percent dilution of the water. 



The extent by which the numbers of bacteria are less than the expected 

 numbers is shown in Figure 5, where the numbers of bacteria found are 



Fig. 4. The dilution of the outfall 

 water, as calculated from the salinity 

 change, and the percentage of bacteria 

 surviving at various distances from the 

 New Bedford Sewer outfall. 



g 5,000 



DltUTIOW, < OtJTPALL WATER 

 Baaed on Sillottr Chang* 



Fig. 5. The number of gas-producing 

 organisms found plotted against the 

 dilution of outfall water as calculated 

 from the salinity change. 



plotted against the dilution of the water. If the bacteria diminished in 

 direct proportion to the dilution of the water a straight-line correlation 

 would be expected since dilution of the water by 50% would lead one to 

 expect half of the original bacterial population. The bacterial numbers 

 are, however, substantially lower than can be accounted for by dilution 

 alone. In the simplified situation at the New Bedford Sewer outfall, there- 

 fore, it is clear that the coliform population disappears much more rapidly 

 than would be expected on the basis of simple dilution by sea water. 



* Each sample of water is considered a mixture of the polluted, fresher water with 

 sea water, i.e. 



XS„ + (l-X)S = Si 



in which X is the fraction of polluted water of salinity S^, and S and Si are the salini- 

 ties of the sea water and of the diluted water sample. 



