44 



PROBLEMS OF LAKE BIOLOGY 



occnrreiu'e of nitrifying' and denitrifying 

 bacteria in Lake Mendota; by Klein and 

 Steiner (1929) to determine the occurrence 

 of urea-splitting, nitrogen-fixing, nitrify- 

 ing, denitrifying, sulphur-oxidizing, and 

 sulphate-reducing bacteria in the lower 

 Lake Lunz; by Steiner (1931) to deter- 

 mine the occurrence of cellulose- and chitin- 

 decomposing bacteria in the same lake; by 

 Williams and McCoy (1935) to determine 

 the occurrence of a variety of physiological 

 groups in the bottom deposit of Lake Men- 

 dota; and by Baier (1935) to determine the 

 occurrence of nitrifying;, denitrifying, ni- 

 trogen-fixing, cellulose- and urea-decom- 

 posing bacteria in several ponds near Kiel. 

 None of these studies have been sufficiently 

 extensive or quantitative to warrant any 

 generalizations regarding the distribution 

 of various physiologic groups. They have 

 served, however, to indicate that all of the 

 important physiologic groups found in soil 

 are also found in lakes. 



Nearly all of the investigators who have 

 used culture methods to study bacteria 

 from the standpoint of limnology have col- 

 lected their samples with one or another 

 modification of the apparatus introduced 

 by Russell (1892). Test-tubes are drawn 

 out to a capillary, partially evacuated, and 

 sealed. They are clamped in a holder at- 

 tached to a line, lowered to the desired 

 depth, and a messenger is dropped which 

 breaks the capillary and allows the sample 

 to be sucked into the tube. Such an appa- 

 ratus useful in studying lakes has been 

 described and illustrated by Wilson (1920). 



Culture methods are the onlj^ ones so far 

 developed which are suitable for the study 

 of bottom muds. Because bottom deposits 

 contain much larger numbers of bacteria 

 than do the lake waters, bottom samples 

 must be more highly diluted for quantita- 

 tive studies; otherwise cultures are made 

 by the same techniques as for water. Hen- 

 riei and McCoy (1938) have discussed and 

 described equipment for collecting bottom 

 samples for bacteriological analysis. 



Since cultural methods require elaborate 

 apparatus and large amounts of glassware 

 not readily available in field laboratories, 

 and especially because no single medium 



Avill permit of a growth of more than a 

 fraction of the species of bacteria, it is 

 natural that attempts toward a more direct 

 study of the water bacteria should be made. 

 ExceiDt for highly polluted waters, the bac- 

 teria occur in numbers too small to permit 

 a direct microscopic enumeration or obser- 

 vation ; they must first be concentrated. 

 Snow and Fred (1926) attempted to do this 

 by centrifuging after adding aluminum 

 hydroxide as a flocculating agent. Cho- 

 lodny (1929) filtered the water samples 

 through collodion membranes, examining 

 smears of the sediment which collected 

 on the filter. Kusnetzow and Karsinkin 

 (1931) evaporated water samples under 

 diminished pressure at 35° to 40° C. The 

 results of various studies based upon these 

 methods have been reviewed by Bere 

 (1933) who used the method of Kusnetzow 

 and Karsinkin on a study of various Wis- 

 consin lakes. 



Such a method includes in the count both 

 living and dead bacteria, and does not dis- 

 tinguish between bacteria M^ashed into the 

 lake from the surrounding land and those 

 active in the water. All of these methods 

 require considerable manipulation which 

 may introduce errors. With evaporated 

 samples, the occurrence of particles of pre- 

 cipitated mineral matter may obscure the 

 field, especially in the case of hard waters. 

 In all cases particles of debris hardly dis- 

 tinguishable from bacteria may occur. The 

 figures published by Kusnetzow and Kar- 

 sinkin, millions per cubic centimeter, are 

 much too high for an unpolluted lake ; such 

 quantities of bacteria would make the 

 water obviously turbid. These methods 

 have all served to show that bacteria are 

 more numerous in the water than plate 

 counts would indicate, the microscopic 

 counts running from 8 to 4,000 times as 

 high as the plate counts. They have not, 

 however, provided data which permit of 

 generalizations regarding the distribution 

 of lake bacteria. 



A fourth method for studying lake bac- 

 teria was introduced by Naumann (1925) 

 for the study of the morphology of certain 

 water bacteria, and adapted independently 

 by myself (Henrici 1933), and by Kar- 



