ZoBell — 206 — Marine Microbiology 



the factors which influence oxygen consumption by bacteria in Lake 

 Mendota. 



Other things being equal, bacteria are generally more abundant in the 

 epilimnion than in the hypolimnion of fairly deep, well stratified lakes. 

 The differences are not marked. Differences in the abundance of bacteria 

 at different depths disappear entirely during overturns and are non- 

 existent in shallow lakes. Only slight or no differences in the vertical 

 distribution of bacteria were observed by Feed et al. (1924) in Lake 

 Mendota and by Graham and Young (1934) in Flathead Lake, Montana. 

 Progressive decreases were found by Kusnetzow and Karzinkin (1931) 

 in Lake Glubokoje and in various lakes in northeastern Wisconsin by 

 Bere (1933). 



Additional observations by Kleiber (1894), Minder (1920, 1927), 

 Pfenniger (1902), DuGGELi (1924), Fred et al. (1924), Ruttner 

 (1932), Zm (1932), and others on the factors influencing the distribution 

 of bacteria in lakes are recorded in Chapter V. 



The densest bacterial populations are almost invariably found in bot- 

 tom deposits. Henrici and McCoy (1938) counted from a few thousand 

 to 500,000,000 aerobes per ml. of lake mud (see Table XXVII on page 

 92). According to Henrici (1939), the bacterial content of the overlying 

 water is no criterion of the number or kinds of bacteria in bottom deposits. 

 The observations of Williams and McCoy (1935), Kusnetzow (19356), 

 RuBENTSCHiK and GoicHERMAN (1935), DtJGGELi (1936), and Car- 

 penter (1939) on the numbers, kinds, and activities of bacteria in lake 

 deposits are noteworthy (see Chapter VI). 



The seasonal distribution of bacteria in lakes is influenced by changing 

 water temperatures, plankton pulses, overturn, runoff, etc. As in the sea, 

 the abundance of bacteria in lakes responds more quickly to changes in 

 organic content than to any other environmental factor. Therefore most 

 seasonal fluctuations can be traced either directly or indirectly to factors 

 which influence the quantity and quality of organic matter. In about 

 half of the lakes examined by Bere (1933) the bacterial content was quan- 

 titatively proportional to the organic and inorganic nutrients in the water, 

 and in about one-third of the lakes the bacterial content was proportional 

 to the organic content alone. The relation of bacteria to the cycle of 

 organic matter in lakes has been reviewed by Waksman (1941a). 



Some of the ways in which aquatic fungi influence the transformation 

 of organic matter are outlined by Weston (1941). The activities of para- 

 sitic and pathogenic fungi are of considerable importance in aquatic 

 biology. Bacteria are also known to infect fresh-water organisms, some 

 of which are of economic significance. 



The studies of Klein and Steiner (1929) and DtJGGELi (1924, 1934) 

 on Swiss lakes, those of Fred et al. (1924) on Wisconsin lakes, those of 

 Kusnetzow (1935/^ 1939) on Russian lakes, and those of Baier (1935) on 

 German lakes are illustrative examples of the results which may be ex- 

 pected to accrue from intensive hydrobacteriological investigations of 

 particular bodies of water. After making a survey of the numbers of 

 various physiological types of bacteria throughout the lake in question, 

 these workers undertook to estimate bacterial activity in situ. This was 

 achieved either by controlled experiments in which concentrations, tem- 

 perature, and other environmental conditions were designed to simulate 

 the natural environment, by comparing the abundance of various physi- 

 ological types of bacteria present from time to time with the chemical 

 composition of the water, or bv a combination of both methods. 



