359 



instance but two, Oct. 14 and 29. In the first of these, silt-laden 

 flood waters in the river, but not in the lake, interrupt the par- 

 allelism. In the second instance the production of the lake de- 

 clines and that of the river rises — again as a result of the prior 

 flood conditions, as will appear on a comparison of the sequelae 

 of the June and August floods in the two bodies of water. In 

 these, as also in the October flood, there are indications that the 

 rising plankton pulse common to both is temporarily suppressed 

 in the river and continues undisturbed and reaches an earlier 

 culmination in the lake, but only a delayed one of slight ampli- 

 tude in the stream. 



The average production in the lake in the last four months 

 of the year exceeds that in the river by 52 per cent, and in five 

 of the seven collections. 



The comparison of production in Quiver Lake and the Illi- 

 nois River in 1896 is very instructive in several important par- 

 ticulars. In the first place, both the relative and absolute pro- 

 ductivity of the lake has increased, rising from 1.08 and .78 

 cm. 3 in 1894 and 1895 to 2.59, an increase of two- to three-fold. 

 The ratio of productivity in the lake to that in the river in 1894 

 was 1 to 2.3; in 1895, 1 to 4.1 ; while in 1896 it falls to 1 to .45. The 

 low average in the river is, as has been shown, the result of the 

 repeated flushing by storm waters. The increase in the lake is 

 due to the higher levels and increased impounding function, 

 and to the actual and relative decrease in its vegetation. The 

 combined result of the operation of these factors is that in this 

 year the lake waters cease to be diluents of the channel plank- 

 ton and become sources of enrichment. Considering the areas 

 of their respective drainage basins, and basing calculations on 

 the yearly averages, the net result of the contributions of Quiv- 

 er Lake is a rise in the plankton content of channel waters 

 from 1.16 cm. 3 per m. 3 to 1.18 — an increase of a little less than 

 2 per cent. 



Not only was the average production in the lake (2.59) 

 greater than that in the stream (1.16), but individual collec- 

 tions upon coincident or approximate dates exhibit the same 



