S80 



These factors, combined with changes in production in the 

 river, vary the relation which these lakes bear to production in 

 channel waters. In general, in times of low water and domi- 

 nance of vegetation the outflow from these lakes is a diluent of 

 channel plankton, but during the run-off of impounded back- 

 waters or in years of higher levels and less vegetation it serves 

 to enrich channel waters. 



The river and the two lakes exhibit in common a very 

 marked similarity in the seasonal movement in production. 

 The recurrent pulses, which may be traced whenever collections 

 are of sufficient frequency, coincide closely in their location 

 but exhibit considerable local differences in their amplitude. 

 This similarity is greatest. when local environmental factors, 

 such as vegetation and tributary waters in the lakes and sewage 

 contamination and recent flood water in the river, are least op- 

 erative, and is diminished or obscured as these factors come 

 more into action. The diversity, as shown in the differing am- 

 plitudes of the pulses of production and in the divergences and 

 interruptions in their rise and decline, can generally be traced 

 to the preponderance of some local factor or factors above 

 named. 



The similarity in the seasonal movement of production is 

 all the more marked when the striking differences of the three 

 localities in question are considered and the general instability 

 of the whole environment is borne in mind. The changes in 

 the plankton content of the river, — turbid and fouled by sewage, 

 traversed continuously by a considerable current, and scoured 

 repeatedly by flood, — of Quiver Lake, — with gentle current,clear 

 spring-fed waters, and greater or less, but always considerable, 

 vegetation,^and of Dogfish Lake, — with tranquil, almost cur- 

 rentless waters, without access of tributary contributions, and 

 also with considerable vegetation, — all exhibit a harmony that 

 compels us to admit the potency of those general factors of the 

 environment common to all — their climatic and geographical 

 surroundings, which determine the amount and distribution of 

 the light and heat, and the chemical constituents of the medi- 



