321 
that the production in this year exhibits a monthly average 
(5.95 em.*) 5 to 25 times (see table on p. 292) that foundin the 
same month in other years. 
The decline at the close of this pulse to .06 em.’ on Noy. 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 
eyclic 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°-+. 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 produe- 
tion is, then, a period of readjustment between summer and 
winter conditions. Again, in the chemical conditions the 
pulse of the nitrites (Pl. XLIV.) and chlorine, and the steady 
rise in free ammonia may indicate conditions which compelled - 
a readjustment of the fluviatile population and resulted ina 
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, 
