THE DISTRIBUTION OF BACTERIA IN LAKES 



55 



weed, bed, particularly at tlie l)()tt()ni. Be- 

 yond the weed bed tlie miinbers are smaller 

 at the bottom. 



Again it is seen tliat the oecnrrenee of 

 plants alfeets markedly the distribution of 

 the periphytie bacteria; both the floating- 

 plankton and the rooted plants near shore 

 exert an influence. A comparison of the 

 data presented in Table I with those shown 

 in Table II will emphasize this influence of 

 the plant life upon the bacteria. Table II 

 shows the results of a similar but less exten- 

 sive transect of Lake Mendota carried, out 

 in 1936. The locations of the stations are 

 shown on the map (Fig. 2) . The shore here 

 is also a wave-swept sandy beach. There 

 are no rooted plants in the zone studied, nor 

 was the lake blooming. The periphytie bac- 

 teria are quite uniformly distributed, save 

 for an increase at the very shore. 



It has been noted that in Lake Alexander 

 the numbers of bacteria in the bottom de- 

 posit are much greater in the sheltered lit- 

 toral stations than in the open lake. Klein 

 and. Steiiier (1929) found somewhat smaller 

 numbers in the littoral stations at lower 

 Lake Lunz. The stations were on an ex- 

 posed shore, but rooted plants were present. 

 Henrici and McCoy (1938) found a rather 

 marked decrease in the bottom bacteria of 

 Lake Mendota as the shore is approached. 

 Samples were taken from the stations indi- 



TABLE VII 



Shoreward Distribution of Bottom Bacteria, 

 Lake Mendota. 



* Averages of counts at all levels to 18 cm. 



cated in Fig. 2, and the results are summar- 

 ized in Table VII. Here the shore is sandy, 

 and as the amount of black humus-like mate- 

 rial becomes less abundant in the bottom 



deposit, tlie bacteria decrease in numbers. 

 The decrease of both humus and bacteria in 

 the bottom deposit in these observations is 

 to be attributed to wave action. 



Summary of Horizontal Distribution. 

 The shoreward distribution of bacteria is 

 profoundly affected by the occurrence of 

 rooted aquatic plants. In sheltered bays 

 periphytie bacteria, bottom bacteria, and 

 bacteria cultivated on agar plates are much 

 more abundant than in the open lake. Even 

 on an exposed shore periphytie bacteria are 

 somewhat more numerous if there is a weed 

 bed in the deeper Avater near shore. Bac- 

 teria in the bottom deposit decrease in num- 

 bers from profiindal to littoral stations 

 where there are no aquatic plants and the 

 bottom becomes sandy toward shore. 



Seasonal Distribution of Bacteria 



Only a small amount of published investi- 

 gations of lake bacteria has been sufficiently 

 extensive or continuous to furnish informa- 

 tion on seasonal variations. In particular, 

 winter observations are lacking. 



Pfenniger (1902) observed a summer 

 minimum, with spring and autumn maxima, 

 in Lake Zurich. This was confirmed by 

 Minder (1920) who attributed the decrease 

 during the summer to the action of sunlight. 

 A summer minimum was noted also by 

 Ruttuer (1932) at Lake Lunz. and by Gra- 

 ham (193-t) in Flathead Lake. Zih (1932) 

 noted, in addition, a winter minimum at 

 Lake Lunz. Pfenniger believed that the 

 death of plankton organisms in the autumn 

 was responsible for an increase of the bac- 

 teria at that time. Ruttuer suggested that 

 the semiannual turnover of the lakes was 

 responsible for the spring and aiitnmn 

 maxima, Init noted also that the highest 

 counts coincided with the period of melting 

 snows. Graham considered the melting 

 snows of spring and the rains of autumn to 

 be responsible for increases of bacteria, the 

 bacteria being washed into the lake from the 

 surrounding land. It will be noted tliat 

 these observations were all made on high 

 mountain lakes, presumably oligotrophic in 

 type. 



Fred, Wilson, and Davenport (1924) con- 



