335 



month that the greater part of the autumnal decline in temper- 

 ature occurs, the total reduction being 27°, at the rate of about 

 1° per day. This, with the other environmental changes above 

 noted, necessitates considerable readjustment on the part of 

 the plankton to the new conditions. We find here as in corre- 

 sponding conditions in other years a decline to a minimum pro- 

 duction, which in 1898 lowers the monthly average to .24 cm. 3 

 per m. 3 , the least recorded for this month in any year (see table 

 on p. 292). The average temperature of surface waters in October, 

 1898, is about 56.4° — about the average, and 8° less than in 1897, 

 when production was much higher (5.95 cm. 3 ). It is 2° higher than 

 in 1895, when production was double (.57) that in 1898. I at- 

 tribute this least production in 1898 in part to the unstable con- 

 ditions in the month prior, and in part to the cumulative effect 

 of the rapid decline in temperatures, which exceeds by 30 per 

 cent., or more, that in other years of our records. The rapidity 

 even more than the extent of the change seems to be the po- 

 tent factor in depressing production. It is significant that this 

 most complete suppression of plankton production occurs at 

 this season of greatest change in environmental conditions. 



The November pulse has a duration of 35 days, — from Nov. 

 8 to Dec. 13, — with a maximum production of 1.26 cm. 3 on Dec. 

 6. Its mean falls on Dec. 2, 43 days after that of the preceding 

 pulse. This was a month of continued rise to a maximum of 

 8.7 ft. on the 25th, after which the river fell. The total move- 

 ment was 3.1 ft. This autumn flood brought with it from bot- 

 tom-lands and prairies a load of silt composed largely of the 

 summer's growth of vegetation, as well as the accumulated ref- 

 use from industrial establishments on the banks of the stream 

 above our location. The silt thus reaches the unusual amount 

 of 22.18 cm. 3 per m. 3 on Nov. 22. The chlorine and free ammonia 

 decline with the dilution of the sewage caused by flood, the ni- 

 trates continue their autumnal rise, and other forms of nitro- 

 gen remain rather constant. The net temperature decline is 

 only 10°, reaching, however, almost the winter minimum. There 

 is no rapid decline in temperature, and the hydrographic 



