ZoBeU 



66 — 



Marine Microbiology 



As a matter of fact, a large number of microorganisms are usually asso- 

 ciated with the surface film of water. The surface film serves as a mechan- 

 ical support for organisms and miscellaneous particulate materials. Or- 

 ganisms which are associated with the surface film of water are termed 

 neuston. Welch (1935) describes various kinds of neuston found in fresh- 

 water lakes. In perfectly calm water the present author has often found 

 two to four times as many bacteria concentrated in the surface film of 

 water as in that collected a few centimeters below the surface, but more 

 often no such relationships can be demonstrated. 



The bacterial neuston is believed to have an influence on the surface 

 tension of water. Occasionally during mild sea breezes, slicks have been 

 observed at considerable distance from land in which the surface tension 

 of the water was found to be only 65 to 68 dynes/cm. as compared with 

 74 to 75 dynes/ cm. for normal sea water. The topmost layer of water 

 from these slicks usually contains many more bacteria than the surround- 

 ing water which is more readily ruffled by the wind. The cause of the 

 increased bacterial populations in these slicks is still undetermined, but 

 they are believed to be responsible for the depressed surface tension. 



As a rule, there are no differences with depth in the abundance of bac- 

 teria in the topmost 5 or 10 meters of sea water from the open ocean at 

 any time of day or at any season of the year. The data recorded in Table 

 XVII are typical of several investigations made at the Scripps Institution 

 on the vertical distribution of bacteria in calm sea water. 



Table XVII. — Average number of bacteria per ml. of sea water collected at various depths at 

 La Jolla, California {from data by ZoBell and McEwen, 1935): — 



Below the topmost 5 to 10 meters (or more in turbulent seas) the bac- 

 terial population generally increases with depth to 25 or 50 meters and 

 then decreases in water at stations off the coast of southern California. 

 Below depths of 200 meters the bacterial population is rather sparse, plate 

 counts rarely revealing the presence of more than 10 bacteria per ml. In 

 many samples rigorously collected from depths exceeding 200 meters, pop- 

 ulations of fewer than one bacterium per ml. have been detected by cul- 

 tural procedures at stations where hundreds to thousands of bacteria per 

 ml. have been demonstrated in the euphotic zone. Thousands to mil- 

 lions of bacteria have been demonstrated per ml. of water taken at the 

 bottom regardless of the depth of the overlying water. The curve in Fig. 

 7 depicts the general vertical distribution of bacteria based upon the 

 average results obtained at more than a score of deep-water stations oflf 

 the coast of southern California. Curves for individual stations differ 

 markedly. 



Schmidt-Nielsen (1901), Bertel (191 2), and Foyn and Gran 

 (1928) have observed that the bacterial population of sea water collected 

 at different stations in the Atlantic Ocean increased from the surface 



