52 



PROBLEMS OF LAKE BIOLOGY 



But the counts are distinctly higher in the 

 top 3 meters. This has been observed re- 

 peatedly in this lake at several stations dur- 

 ing midsummer, not in other lakes which 

 I have studied. It is correlated with the 

 enormous production of blue-green algae. 

 The lake regularly blooms in August, and 

 the floating plankton is largely concen- 

 trated at the surface. Below the 3 meter 

 level, there are no striking or consistent 

 variations until the very bottom is reached, 

 where there is a decrease that has also been 

 noted in other observations in this lake. 



Lake Mendota is stratified, the thermo- 

 cline lying at about 12 meters depth. While 

 counts from the epilinniion average higher 

 than those from the hypolimnion, the differ- 

 ence is slight and probably not significant. 

 More significant differences are found in 

 Muskellunge Lake, where the thermocline 

 occurs at 8 meters depth; below this level 

 the counts are much less than those in the 

 upper layers. There is also a much sharper 

 difference in temperature between the epi- 

 linniion and the hypolimnion than in Lake 

 Mendota. 



A similar distribution of bacteria is found 

 in Trout Lake, the deepest that has been 

 studied. Here the periphytic bacteria are 

 much less numerous below the thermocline, 

 and there is a correspondingly great differ- 

 ence in temperatures. Crytal Lake, which 

 is shallower and with less sharp stratifica- 

 tion, shows an almost uniform distribution 

 from top to bottom, while Weber Lake, still 

 shallower, shows a progressive increase to- 

 ward the bottom. 



The two dystrophic lakes, Mary and Hel- 

 met, show striking vertical distributions of 

 the periphytic bacteria. In both of these 

 the thermocline is at the surface ; the tem- 

 perature drops very rapidly. Periphytic 

 bacteria are relatively abundant at the sur- 

 face, almost completely absent in the lower 

 levels; even after four weeks of immersion 

 they were too few to count. 



From the data presented it seems evident 

 that periphytic bacteria show a marked de- 

 crease below the thermocline in those lakes 

 Avhich are sharply stratified (Glubokoje, 

 Muskellunge, Trout, Mary, and Helmet) 



while they are fairly uniform in distribu- 

 tion in shallower or less sharply stratified 

 lakes (Alexander, Mendota, Crystal, and 

 Weber). The vertical variations are prob- 

 ably due almost entirely to temperature; 

 they have been observed in eutrophic, oligo- 

 trophic, and dystrophic types, and are more 

 marked with greater temperature differ- 

 ences. 



In the data presented there are several 

 anomalous counts recorded. Thus in Mus- 

 kellunge Lake, although consistent results 

 were obtained at other levels, the slides at 

 10 meters were almost blank ; in Trout Lake, 

 unusually high figures were obtained at 18 

 and 20 meters, although at all other levels 

 in the hypolimnion the counts were low ; an 

 anomalous high count is noted at 6 meters 

 in Crystal Lake. Such local anomalous 

 counts have been noted in other observa- 

 tions. They are of such a magnitude that 

 they must be statistically significant, but I 

 am at a loss to explain them. They may be 

 clue to the same causes which give rise to 

 local variations in turbidity, discovered and 

 described by Whitney (1938) and desig- 

 nated by him "microstratification." It is 

 noteworthy that Whitney found local in- 

 creases in turbidity to be associated with in- 

 creases in organic matter and bacteria 

 (plate counts). 



Bacteria are more numerous in the bottom 

 deposits of lakes than in the water. From 

 data published by Henrici and McCoy 

 (1938) it can be computed that the average 

 bacteria per cubic centimeter in bottom de- 

 posits was from 12 to 680 times as great as 

 the number per cubic centimeter in the 

 water; these figures are based upon plate 

 counts from nine different lakes. The 

 greater number of bacteria in the bottom is 

 probably due to several different factors, the 

 most important being the tendency of the 

 bacteria to be adsorbed by or otherwise at- 

 tached to solid particles in the water, and 

 to be carried by these particles to the bot- 

 tom. Kleiber (1894) noted that large num- 

 bers of bacteria brought into Lake Ziirich 

 by inflowing streams extended only a short 

 distance into the lake, and attributed their 

 disappearance to sedimentation, and this 



