305 



conditions, and the result is a pulse of considerable amplitude, 

 moreover, one not attained in any other year in channel plank- 

 ton ; a fact whose significance is apparent when we find (PI. 

 VIII. — XII.) that no other year combines to the same extent 

 stability of hydrographic and thermal factors. The relative 

 production in different years (see table on p. 292) bears upon 

 the point in question, the monthly mean (3.02) being from 150 

 to 3 times as great as that reported for other years. The chem- 

 ical conditions attending this remarkable plankton production 

 (PI. XLIII.) are those following increased sewage contamina- 

 tion, namely, a rise in nitrates, free ammonia, and chlorine. 



The December pulse has a duration of 21 days, — from De- 

 cember 4 to 25, — the limits being taken from the statistical data 

 (PI. LI.), which are based only on catches at intervals of 5 to 7 

 days. Its maximum amplitude (2.60) occurs on the 20th, and 

 its mean falls on the I6th, 24 days after that of the preceding 

 pulse. The first 18 days of the month are relatively stable, 

 with a movement in levels of only 1.05 ft. and stable minimum 

 temperatures under the ice sheet. During this period a slight- 

 ly developed pulse begins its course (cf. also PI. LI.), but its 

 apex does not rise much above the level of previous produc- 

 tion. It is noteworthy that this takes place beneath the ice 

 sheet which covered the upper river during the fortnight pre- 

 ceding the flood. It is in this month that the contamination 

 noted in November reaches its maximum (PL XLIII.), at least 

 as shown by nitrates and free ammonia. The chemical condi- 

 tions thus favor a continuance of the productive activity of 

 the previous month. On the 19th heavy general rains started 

 a flood of unusual magnitude which continued to rise, culmina- 

 ting at 12.6 ft. at the end of the month. This raised the temper- 

 ature about 9°, brought in an immense load of silt, flushed out 

 the plankton, and increased the rate of the current so as to 

 greatly reduce the time for breeding. The first two days of 

 rising water did not materially change the quantity of plank- 

 ton per m.^, indicating a considerable rise in production had not 

 the flood occurred. By the 25th, however, the flood waters had 



