334 



The nitrates are at a minimum (PI. XLV.) during this 

 month, and the sewage contamination is shown by the high 

 nitrites and increase in chlorine and free ammonia. The move- 

 ment in these substances is correlated with that in the river 

 levels rather than that of the plankton. The maximum ampli- 

 tude of this pulse follows at an interval not exceeding a week 

 the heat pulse of the last part of July, and its slight decline co- 

 incides with a period of lowered temperatures. 



The September pulse has a duration of 42 days, — from Aug. 

 23 to Oct. 4, — with a maximum amplitude of .95 cm. 3 per m. 3 on 

 the 20th. Its mean falls on the 12th, 34 days after that of the 

 preceding pulse. The movement in levels in the month of Sep- 

 tember is 5.9 ft., and no less than five small rises appear in the 

 records during this pulse (see page 161). The result is a con- 

 siderable fluctuation in nitrites, chlorine, and free ammonia, — 

 all of which tend to run high at this season, — and a depres- 

 sion of the plankton to a continued low level of production, .69 

 cm. 3 per m. 3 being the monthly average. The production in 

 other and more stable years is from two- to thirteen-fold great- 

 er. In 1896 alone, a year of even greater September disturb- 

 ances, does the production fall below that of 1898. The net 

 decline in temperature is about ten degrees, and the production 

 at the decline of this pulse is lower than that in other instan- 

 ces, save one, since the preceding April. The cold wave in the 

 middle of the month (PL XII.) with a temporary decline of 20° 

 coincides with a slight decline in production. 



" The October pulse has a duration of 35 days, — from October 

 4 to Nov. 8, — with a maximum amplitude of .42 cm. 3 per m. 3 on 

 the 18th. The mean falls on the 20th, 38 days after that of 

 the preceding pulse. This was a month of more stable hydro- 

 graphic conditions, though the flood at its close brings up the 

 total movement to 3.7 ft. Considerable movement in chemical 

 conditions also occurs, as this is the beginning of the period of 

 readjustment to lowered temperatures. The nitrates rise slow- 

 ly, the nitrites fall to the winter minimum, and the chlorine 

 and free ammonia fall and rise with river levels. It is in this 



