56 



PROBLEMS OF LAKE BIOLOGY 



ducted extensive observations over three 

 years on Lake Mendota. Tlie results varied 

 from year to year ; there was a summer max- 

 imum in 1920, an autumn maximum in 1921, 

 and a spring maximum in 1922. They dis- 

 cuss the complexity of factors which may 

 influence seasonal variations in the plate 

 counts, and emphasized the importance of 

 rainfall ; the counts were apparently influ- 

 enced markedly by the washing of bacteria 

 into the lake from the drainage basin. 



The investigations of Domogalla, Fred, 

 and Peterson (1926) have provided some in- 



Sulphate reduction occurred only during 

 the winter and early spring. 



Tlie author has previously published 

 (Henrici 1937) a report on seasonal fluctua- 

 tions of bacteria in Lake Alexander. The 

 essential data are shown in Fig. 3, repro- 

 duced from that paper. The curves show 

 the numbers of bacteria in plate counts, 

 perij^hytic bacteria, and the volume of net 

 l^lankton. Samples were collected at Sta- 

 tion 1 (Fig. 1). The reader is referred to 

 the original paper for details of technique. 



Lake Alexander is unstratified, therefore 



60- 



50- 



laoo 



1600 



1400 



40- 



1200 



2 ^ 



-^ S 



J 30 ,,1000 



p. ^ 

 P. 



aoo 



20- 



600 



400 



10- 



200 







■ Microscopic courvU / 



Bacteria per sa. mm. per day i 



■ Plor\kton 



Cc. of pediment from. 

 50 nxeter haul of net 



■ Plate counts 



Bacteria per cc. of water 



1933 



Ice wer\t out April 20 

 Lake froze Nov. 12 



-600 



500 



400^ 



300 



-200 



100 



September October 



Fig. 3. 



formation on seasonal variations in the ac- 

 tivities of bacteria concerned in the nitrogen 

 cycle. They found that nitrification was 

 most intense during the late summer and 

 autumn, when ammonia and organic nitro- 

 gen reached the highest concentrations; 

 while nitrate reduction was least during the 

 summer. Klein and Steiner (1929) found 

 reverse conditions at Lake Lunz. Nitrifica- 

 tion was most active in the winter, at which 

 season denitrification was reduced to zero. 



free from the effects of turnover. The ob- 

 servations were made during a drought 

 year when the lake was singularly free from 

 surface drainage. The lake is highly- pro- 

 ductive and blooms in the summer, in this 

 respect quite different from the high moun- 

 tain lakes mentioned previously. 



It will be seen that the plankton pulses 

 were an important factor in determining 

 the occurrence of seasonal variations of the 

 bacteria. Both sets of counts of bacteria 



