THE DISTKIIiUTIOX OF BACTERIA IX LAKES 



53 



observation lias been repeated by several 

 other later writers. Rubentsehik. Roisin, 

 and Bieljansky (1936) t'onnd that lake bac- 

 teria (especially those cultivated from bot- 

 tom deposits), could be adsorbed from aque- 

 ous suspensions by bottom ooze from lakes. 



The distribution of bacteria in bottom 

 deposits can be determined only by plate 

 cultures. Bacteria are not deposited upon 

 slides in numbers at all commensurate with 

 their abundance in the mud, and because of 

 the larg-e amount of debris, direct micro- 

 scopic counts are impossible. Henrici and 

 McCoy (1938) have reviewed the published 

 literature on the distribution of bacteria in 

 bottom deposits. They found that bacteria 

 are most abundant at the mud-water inter- 

 face. They decrease with deptli in a rather 

 irregular manner with different samples, 

 but this irregularity is due to accidental 

 variations or to technical errors. When 

 numbers of samples are averaged, the bac- 

 teria are found to decrease with depth in a 

 regular manner; the numbers plotted 

 against depth in the bottom form a loga- 

 rithmic curve. The rate of decrease is 

 therefore constant. 



The larger numbers of bacteria in the bot- 

 tom deposit, determined entirely by plate 

 counts, do not necessarily indicate that their 

 activities are greater than in the water 

 above. Since the bottom is usually very 

 cold, and anaerobic, it is quite possible that 

 the bacteria there are relatively inactive. 

 The concentration of organic matter is, of 

 course, much greater in the bottom deposit 

 than in the water ; but this is the less readily 

 decomposed organic matter, since presum- 

 ably the more readily decomposed mate- 

 rial will be made soluble shortly after the 

 death of aquatic organisms, i.e., before or 

 immediately after they have sunk to the bot- 

 tom. The logarithmic curve of decrease of 

 bacteria in the bottom has been compared 

 by Henrici and ]\IcCoy to the logarithmic 

 death curve in disinfection experiments ; 

 they suggest that it may indicate that bac- 

 teria are active only at the very surface of 

 the bottom, dying below. A consideration 

 of these facts will indicate that we are as 

 yet unable to compare the relative impor- 



t.mce of bacteria in the water and in the 

 bottom deposit from the standpoint of their 

 activities. 



Summary of Vertical Distribution. Bac- 

 teria may be more numerous at the surface 

 of tlie watei- in some lakes; this is probably 

 due to the accumnlation of floating plank- 

 ton at the surface, and is most apparent in 

 lakes that bloom. Plate counts do not show 

 marked differences between the epilimnion 

 and hypolimnion, but such differences are 

 noted in counts of bacteria on submerged 

 slides, in the case of sharply stratified lakes ; 

 they are probably due to temperature dif- 

 ferences. There is only slight evidence that 

 bacteria increase in the thermocline. They 

 may show sharp local variations in vertical 

 distribution associated with microstratifica- 

 tion. Bacteria are more abundant in the 

 bottom deposit than in the water, and most 

 abundant at the mud-water interface, de- 

 creasing at a constant rate below this level. 

 Their abundance in the bottom is probably 

 due mainly to sedimentation. 



HORIZOXTAL DlSTRIBUTIOX OF BACTERIA 



Very little previous work has been pub- 

 lished on the horizontal distribution of bac- 

 teria ill lakes. Pfenniger (1902) found 

 the bacteria to be uniformly distributed in 

 the surface waters at various stations in 

 Lake Ziirich. Klein and Steiner (1929) 

 found somewhat higher plate counts from 

 surface water samples taken near the banks 

 than in the open water of lower Lake Lunz. 



I have collected considerable data on the 

 horizontal distribution of bacteria in Lake 

 Alexander. Both plate counts and counts 

 of peripliytic bacteria on submerged slides 

 are available. During the summer of 1933 

 collections were made at seven stations, 

 those marked with numbers on the map 

 (Fig. 1). During the summer of 193-1 col- 

 lections were made at the stations marked 

 with letters on the map, to provide a sort of 

 bacteriological transect of the lake. 



Stations 1 and 7 are open-lake stations. 

 Station 2 is in a sheltered bay, at tlie very 

 edge of the water, where a dense mat of 

 Ceratophyllum, Naias. Elodca, and A-arious 

 Potamogetons merge with a marsh of cat- 



