369 



In 1895 the lake was choked with vegetation which the 

 winter flood largely removed and the recurrent floods of the 

 following year reduced somewhat in extent, while higher levels 

 lowered its relative occupation of lake waters. 



PLANKTON PRODUCTION. 

 1895. 



(Table VI., PI. XXX.) 



There were 12 collections in this year, from April to De- 

 cember, averaging 3.25 cm. 3 per m. 3 The average of the 

 monthly averages (see table between pp. 342 and 343) is 3.3 

 cm. 3 to .74 and 6.65 — similar averages for the same period in 

 Quiver Lake and the Illinois River. The maximum collection 

 was made Dec. 19 — a very unusual date for such production. 



A superposition of the planktographs of the river and 

 Quiver and Dogfish lakes for this year brings out some in- 

 structive similarities and differences in the movement of pro- 

 duction. The vernal pulse of April 29, in so far as the data 

 reveal it, is quite similar in all three localities, reaching its 

 greatest development in Dogfish Lake (8.20), where im- 

 pounding action is greatest, and being greater in the river (5.83) 

 than in Quiver Lake (4.57), where, owing to low levels, the 

 proportion of water of recent creek or spring origin is greater 

 than in the channel of the adjacent river. 



The June-July pulse may be found in all three localities, 

 but it is belated and much smaller in the lake waters. This 

 pulse in Dogfish Lake (4.59 cm. 3 per m. 3 ) is less than a sixth of 

 that in the river (30.42), where, in the semi-stagnant sewage- 

 polluted channel waters of unusually low levels, Moina and 

 other Cladocera caused the unusual production. Between the 

 April and June-July pulses the river levels fell 2 ft., to mini- 

 mum stages (PI. XXX.), so that the proportion of creek and 

 spring water in Quiver Lake is probably more than doubled at 

 the later date. This may account in large part for the very 

 low production in Quiver Lake (.02) on July 8, while on July 5 



