Chapter XVIII — 205 — Microbiology of Inland Waters 



water is more or less constant. These indigenous forms are present at all 

 seasons and at all depths. As compared with other organisms commonly 

 studied, the majority of them grow slowly on ordinary culture media, and 

 on plate cultures appear as punctiform colonies. About io° to 25° C. is the 

 optimum for their growth. In general they do not form acid or gas from 

 sugars. They do not curdle milk and the majority of the true lake forms 

 liquefy gelatin slowly. A considerable proportion are chromogenic, but 

 long incubation at comparatively low temperatures is necessary to bring 

 out the deep color. It is in respect to pigment production that the typical 

 water flora of Lake Mendota is most easily recognized. A large number of 

 pure cultures were selected from well isolated colonies on plates poured 

 during the winter, when the true water bacteria were more prevalent." 



Data summarized in Table XXXI on page 114 show that Taylor 

 (1942) found nearly three times as many Gram-negative rods in English 

 lakes as in soil, and twelve times as many Gram-positive rods in soil as in 

 lake water. There were five times as many cocci in soil as in lake water. 

 The chances for transplantation are so great that virtually any soil 

 microorganism may be found in lakes, but apparently only certain types 

 find conditions suitable for their multiplication or prolonged survival in 

 lakes. 



Baier (1935) compared the bacterial species found in three shallow 

 lakes around Kiel, Germany, where each body of water was found to have 

 a more or less distinctive microflora. No attempt was made to isolate new 

 species, but he recognized many common soil forms and several sulfur 

 bacteria. Baier was especially impressed by the differences in the fun- 

 gous flora of soil, lake water, and bottom deposits. He concluded that the 

 failure of soil fungi spores to germinate in aquatic environments is due to 

 their inability to compete with bacteria for the limited supply of oxygen 

 and organic matter. 



The total number of bacteria found in any given lake depends upon 

 the distance from land, depth, season, type of lake, and other factors, 

 many of which are only poorly understood. After reviewing the literature 

 on the subject, Henrici (1939) declared that data are as yet too incom- 

 plete to warrant any general conclusions regarding the distribution of 

 bacteria in different types of lakes. The largest bacterial populations 

 usually occur in eutrophic lakes, which are richer in organic nutrients than 

 are oligotrophic lakes. Dystrophic lakes, which are rich in humus, occupy 

 an intermediate position, although the microflora in many dystrophic 

 lakes differs more in quality than in quantity from that of other types of 

 lakes. The eutrophication or dystrophication of lakes (Welch, 1935) is 

 influenced by bacterial activities and the latter are in turn modified by 

 other factors. This chain of causes and effects further emphasizes the 

 necessity of having detailed information on microbiological activities when 

 attempting to explain the metabolism of lakes. 



One of the distinguishing characteristics of oligotrophic lakes is the 

 disappearance of oxygen in the hypolimnion during the period of stagna- 

 tion. KusNETZOW and Karzinkin (1931), Miyadi (1934), Kusnetzow 

 (1935&), and others have shown that bacterial activity in deeper water or 

 in lake JDottoms is primarily responsible for the depletion of oxygen. Com- 

 bining field and laboratory observations, Liagina and Kusnetzow (1937) 

 calculated that the decrease in the oxygen content of the waters of Lake 

 Glubokoje in Russia could be accounted for by the respiration of bacteria. 

 Similar conclusions were reached by ZoBell (1940a), who investigated 



