50 



PROBLEMS OF LAKE BIOLOGY 



000 per cc) in the thermoeline of the same 

 lake, associated with an oxygen-minimum 

 in that zone. Bere (1933) using- the micro- 

 scopic method of Kiisnetzow and Karsinkin, 

 observed differences in the vertical distribu- 

 tion of bacteria in various lakes of north- 

 eastern Wisconsin. Bacteria were found to 

 be most numerous in the upper 5 meters of 

 Trout Lake, the numbers being uniformly 

 less below that level. Crystal Lake and 

 Silver Lake showed no definite variations. 

 Mary Lake and Nebish Lake showed a dis- 

 tinct progressive increase with depth. 



Nearly all of the studies which have been 

 mentioned were carried out during the sum- 

 mer months in stratified lakes. There is 

 no clear trend apparent. In some in- 

 stances higher counts at the surface may be 

 due to the influence of inflowing water as 

 was suggested by Pfenniger. Except for 

 the peculiar lake studied by Diiggeli, there 

 is no sharp differentiation between the 

 epilimnion and hypolimnion. The author 

 has previously published data (Henrici 

 1936) comparing plate counts and counts of 

 periphytic bacteria at various levels in four 

 lakes. It was found that in some cases the 

 periphytic bacteria showed marked differ- 



ences while plate counts shoAved none. This 

 may be explained by the fact that the plate 

 count represents not only active, but also 

 dormant bacteria. Undoubtedly bacteria 

 are constantly settling from the upper lay- 

 ers to lower ones, although in the latter they 

 may be unable to grow because of tempera- 

 ture, or oxygen, or other relations. The 

 slide counts indicate only bacteria growing 

 in the water. 



Karsinkin (1934) studied the distribution 

 of periphytic bacteria in Lake Glubokoje. 

 The results varied somewhat with the season 

 and with the duration of exposure of the 

 slides. Observations were made at the be- 

 ginning of September when the lake was 

 still stratified. More bacteria were found 

 in the epilimnion than in the hypolimnion; 

 there were two maxima, one near the sur- 

 face and one at the thermoeline. Observa- 

 tions made at the end of September, when 

 the lake had turned over, showed greatly 

 reduced numbers which were uniform from 

 top to bottom. 



In Tables III, IV, and V there are pre- 

 sented data on the vertical distribution of 

 periphytic bacteria in eight of the lakes 

 which I have studied. Data have been se- 



TABLE III 



Vertical Distribution of Periphytic Bacteria Ditring Midsummer Stratification. 



EuTROPHic Lakes. 



* Bacteria per square mm per day deposited upon slides. 

 t Median date of period of immersion. 



