321 



that the production in this year exhibits a monthly average 

 (5.95 cm.-') 5 to 25 times (see table on p. 292) that found in the 

 same month in other years. 



The decline at the close of this pulse to .06 cm.' on Nov. 2 

 reaches the lowest point recorded after the midwinter-flood con- 

 ditions of the previous February. This decline is abrupt and 

 complete, and is followed by a recovery in production of ap- 

 parently normal proportions. The prime cause may lie in the 

 cyclic growth and reproduction of the planktonts in which an 

 "internal" factor may be dominant, or it may be due to the 

 operation of one or more external factors in the environment 

 or to the combined action of internal and external factors. 

 What external factors can be cited to "explain" this abrupt 

 decline in production in the midst of these apparently stable 

 conditions? In the first place, the minimum record (.06 cm.'') 

 was made when the autumnal decline had reached 54° (a little 

 below the yearly average), and after a decline of 25°+ from 

 the summer heat of 80° -f. The cumulative effect of this 

 change in temperature is suggested, and similar declines in pro- 

 duction during or towards the close of the autumnal decline in 

 temperature in other years may be cited in corroboration of this 

 conjecture. The recovery in production in this year in the face of 

 the cumulative effect of further decline may not weaken the force 

 of this conjecture, since it occurs at a time of change in the con- 

 stituent organisms of the plankton. This minimum of produc- 

 tion is, then, a period of readjustment between summer and 

 winter conditions. Again, in the chemical conditions the 

 pulse of the nitrites (PI. XLIV.) and chlorine, and the steady 

 rise in free ammonia may indicate conditions which compelled 

 a readjustment of the fluviatile population and resulted in a 

 temporary decline in production. It is not, however, a simple 

 matter to find corroborative instances in the records. It may 

 be that we have in this marked decline at the close of this 

 pulse an instance of combination of the internal ( cyclic) factor 

 on the part of the constituent organisms and several depress- 

 ing agencies in the environment, whose united effect is this 

 almost complete but temporary suppression in production. 



