571 



duction. The plantographs of this lake are marked by extreme 

 changes in brief time, and by depression in production with 

 the emergence of vegetation. 



12. Thompson's Lake also produces more (8.26) than 

 channel waters, and maintains its higher level of production 

 more generally. Its impounding function and the freedom 

 from access of tributary waters contribute to this result. 



13. Phelps Lake produces the most abundant plankton 

 (22.55) of all our localities, freedom from vegetation and from 

 access of tributary waters, and the highly developed impound- 

 ing function contributing to this result. The maximum pro- 

 duction in our records, 224.5 cm.^ per m.^ was found in this lake 

 on Aug. 23, 1898. This lake is marked by relatively and abso- 

 lutely high production in summer and autumn. 



14. The course of plankton production in channel and 

 backwaters throughout the year exhibits a series of recurrent 

 pulses, culminating in maxima and separated by minima, 

 which give the planktograph the appearance of a series of 

 "frequency of error" curves of varying amplitudes. These 

 pulses generally have a duration of 3 to 5 weeks, and tend to 

 coincide in their location in all localities coincidently exam- 

 ined by us. This similarity in the direction of movement in 

 production amounts quantitatively to 65 per cent, of the possi- 

 ble comparisons in our records. This cyclic movement in pro- 

 duction is plainly influenced, accelerated or retarded, or its 

 amplitude extended or depressed, by environmental factors, but 

 is not itself traceable to any one or any combination of them. 

 A brief interval of examination — not more than one week—is 

 essential to a demonstration of the existence of these pulses. 



15. Area and depth, within limits of our environment, 

 show little relation to plankton production. 



16. Age of the water is an important factor in determining 

 production in streams. Young waters from springs and creeks 

 have but little plankton, and even such tributaries as Spoon 

 River (drainage basin 1,870 square miles) contain but little 

 plankton, principally of more rapidly developing organisms. 



