ZoBell — 86 — Marine Microbiology 



where a marked purification has been observed, the conditions are pre- 

 cisely those which are most favorable for sedimentation. Gainey (1939) 

 emphasized the importance of sedimentation in the purification of water 

 by declaring that ''The decrease in bacterial numbers in water stored in 

 reservoirs, while not entirely due to sedimentation, should probably be at- 

 tributed largely to this factor." He gives substantiating laboratory and 

 field data. Applying Stokes' law of falling bodies, Spitta (1903) esti- 

 mated that 20 to 50 per cent of the bacteria in canal water settle out due 

 to their attachment to gross particles. 



Russell (1891) recognized the importance of sedimentation in the dis- 

 tribution of bacteria in the Gulf of Naples where many more bacteria were 

 found in the bottom deposits than in the superficial strata of water. How- 

 ever, the occurrence in bottom deposits of bacterial species never found 

 in the overlying water convinced him that bacteria multiply in the bottom 

 deposits and are not merely passive transients from the overlying water. 

 Henrici (1939) ascribed the great preponderance of bacteria in lake bot- 

 tom deposits as compared with the number found in the overlying water 

 to several different factors, "the most important being the tendency of 

 bacteria to be adsorbed by or otherwise attached to solid particles in the 

 water, and to be carried by these particles to the bottom." 



Sedimentation is most important in removing bacteria from sea water 

 along the coast, particularly in localities where there is much land drain- 

 age. In such places the precipitation and sedimentation of suspended 

 matter are accelerated by flocculation which occurs when fresh water is 

 mixed with sea water. Sedimentation is believed to be primarily respon- 

 sible for the localization of the pollution of sea water by land drainage as 

 manifested by the rapidity with which bacterial populations decrease with 

 distance from sewage outfalls and from the mouths of rivers. In such 

 places the bacterial population decreases much more rapidly than can be 

 accounted for by dilution alone. 



From the average bacterial content of river water and that of sewage 

 entering the Pacific Ocean, it is estimated that no fewer than 5 X 10^^ bac- 

 terial cells enter the sea from the land along the coast of the North Pacific 

 Ocean each day. Yet rarely does one find terrigenous bacteria in the sea 

 at distances greater than a few miles from the mouths of rivers or sewage 

 outfalls, in spite of water movements which are favorable for the wide- 

 spread distribution of such organisms. The majority of the bacteria are 

 carried to the sea bottom very near the point of entrance to the sea. The 

 principal exceptions to this rule are caused by small quantities of floating 

 solids and by fresh water flowing over the non-turbulent surface of sea 

 water for considerable distances. 



Sedimentation also has a marked influence on the distribution of bac- 

 teria in lakes, as shown by the quantitative studies of Kleiber (1894) in 

 Lake Zurich. Bacteria introduced into the lake in large numbers by in- 

 flowing streams are detectable only for short distances in the lake. The 

 decrease in the bacterial population parallels the rapidly diminishing tur- 

 bidity or cloudiness in the lake induced by the streams. An average of 10 

 bacteria per ml. was demonstrated in lake water a few meters from shore 

 as compared with plate counts exceeding 10,000 per ml. of water in the 

 mouth of the river. Since calculations indicated that this observation 

 could not be attributed solely to dilution, Kleiber considered sedimenta- 

 tion to be the chief cause of the rapid diminution of bacteria where streams 

 enter the lake. 



