THE DISTRIBUTION OP BACTERIA IX LAKES 



59 



TABLE X 



Distribution of Periphytic Bacteria by Depth and Season, Lake Mendota.* 



Numbers of Bacteria Per sq. mm Per Day Deposited upon Submerged Slides. 



Dates Are Median Dates of Period of Immersion. 



1936 



Observations from Jan. 16 to May 4 at Station A ; from June 28 to Sept. 19 at Station 1. 



ton pulses appeared to determine seasonal 

 variations of the bacteria. The activities 

 of periphytic bacteria vary with the tem- 

 perature, but this is more apparent in open 

 water stations than littoral ones. Peri- 

 phytic bacteria were found to be unusually 

 abundant in the bottom meter of Mendota 

 during winter stagnation. There are no 

 adequate data on seasonal variations of 

 bacteria in the l)<)ttom deposits. 



Abundance of Bacteria in Different 

 Types of Lakes 



In spite of numerous exceptions, the 

 classification of lakes into three basic types 

 — eutrophic, olig'otrophic and dystrophic — 

 has remained a useful concept. It is now 

 quite clear that dystrophic lakes are a class 

 apart, but that eutrophic and oligotrophic 

 layes tend to form a graded series, quite 

 distinct at the extremes, but with interme- 

 diate types often difficult to classify. Of 

 the various characteristics used to distin- 

 guish eutrophic from oligotrophic lakes, the 

 disappearance of oxygen in the hypolimnion 

 of the former during summer stagnation is 

 probably the most important. The decrease 

 of oxygen has been attributed to several 

 agents. Alsterberg (1927) thought it was 

 due to the oxygen-consuming capacity of 

 the bacteria growing in tlie sediinented or- 

 ganic matter of the bottom deposit. This 

 has been perhaps the most widely accepted 

 explanation. Kusnetzow and Karsinkin 

 (1931) believed that the respiration of bac- 



teria in the water of the hypolimnion itself 

 was responsible for the decrease of oxygen. 

 This opinion was based largely on the re- 

 sults of their studies of the enormous num- 

 bers of bacteria found by direct microscopic 

 counting of evaporated samples from Lake 

 Beloje and Lake Glubokoje. I have already 

 expressed scepticism regarding the accu- 

 racy of the method. If the figures are cor- 

 rect, the numbers of bacteria in tliese lakes 

 must be much larger than tliose usually 

 encountered in unpolluted waters. Later 

 Kusnetzow (1935) suggested that the de- 

 crease of oxygen in the hypolimnion was 

 due in part to the displacement of oxygen 

 by liydrogen and methane liberated from 

 the bottom deposit througli the activities of 

 anaerobic bacteria, in part to the oxidation 

 of these gases by autotrophic bacteria in the 

 water. It is quite possible that all of these 

 agencies combine to consume oxA^gen. In 

 any case, it appears to be the activities of 

 the bacteria in the lower parts of the lake 

 which are concerned. The subdivision of 

 lakes on the basis of the oxygen content of 

 tlieir hypolimnions is, in fact, a sort of 

 measurement of the organic matter which 

 can l)e oxidized by bacteria, similar to the 

 measurement of the "biological ox.ygen 

 demand" used by sewage bacteriologists. 



Dystrophic lakes differ from the eu- 

 ti'ophic-oligotropliic series in many respects, 

 but from a bacteriological standpoint the 

 most important is the large quantity of 

 peculiar organic matter in the water, oc- 



