388 



of silt on March 30 — with increase in temperatures in January- 

 April, culminating in a vernal pulse in April-May, which in 

 Flag Lake reaches a much higher level (203.52) than elsewhere, 

 culminates later by 7 to 14 days and is not divided into two 

 apices as in the other three localities, but in duration covers 

 the period of two pulses elsewhere. It is further seen in the 

 May-June and August pulses and in the fairly well sustained 

 correspondence in direction of the changes in the September- 

 December period. The most marked disagreement appears 

 with the declines in stage of the river in May and July, when 

 local environmental factors are most potent, and when, also, 

 vegetation is at the height of its relative occupancy of the 

 lakes in question. 



One of the most striking features in the production of this 

 lake, and one not without parallels elsewhere in our records 

 (PI. XXIX., XXXI. ), is the very sudden decline in plankton 

 content after the vernal pulse, namely, from 203.52 cm. 3 per m. 

 on May 2 to 47.7 on the 9th— a decline of 77 per cent, in 7 days. 

 On the 15th it reached the low level of .72, a decline of 98 per 

 cent, in 6 days or of 99.6 in 13 days. The attendant hydro- 

 graphic conditions are not without significance. This pulse 

 (PI. XXXIII.) attains its growth between March 30 (1.02) and 

 May 2 (203.52), in which period the net drop in levels in 

 channel waters is only from 8.1 to 6.9 ft. and the total move- 

 ment only 1.7 ft., while in this protected backwater the fluctu- 

 ations are probably somewhat lessened, as will be seen in the 

 fact that the depth in the lake changes only .5 ft. to 1.2 ft. in 

 the channel. The pulse thus rises in stable conditions. 



The decline of the pulse takes place between May 2 and 23 

 from 203.52 cm. 3 per m. 3 to .12. In this time levels fall from 7.1 

 to 4.9 (see p. 159) on the 17th and rise again to 7.2 on the 23d. 

 The decline in production from the 15th (.72) to the 23d (.12) 

 is so small a part of the total that its significance in the present 

 connection is slight, and the rise in levels has probably not had 

 time to materially affect the lake. The hydrographic influences 

 potent in the decline in production have been operative prior 



