572 
This barren water, impounded for 10-30 days in backwater res- 
ervoirs such as Phelps Lake, develops an abundant plankton. 
The rate of run-off and replacement of impounded waters de- 
termines to some extent the amplitude of production. This is 
greatest where run-off is least and rate of renewal slowest. 
17. Fluctuations in hydrographic conditions constitute 
the most immediately effective factor in the environment of the 
potamoplankton. Rising levels usually witness a sharp decline 
in plankton content (per m.*) as barren storm waters mingle 
with or replace plankton-rich waters of channel and reservoir 
backwaters. Falling levels are periods of recovery and increase 
in plankton. Stability in hydrographic conditions conduces to 
rise in production at all seasons of the year, and instability is 
always destructive. Winter floods tend to lower plankton 
production; spring floods increase it. 
18. Temperature affects production profoundly. Below 
45° the plankton content in the river is only about 9 per cent. 
of that present above this temperature, and in backwaters but 
29 to 40 per cent. Minimum production is at times of mini- 
mum temperature. The vernal pulse in production attends the 
vernal rise in temperature and culminates at about 60°—70°. 
With the establishment of the midsummer temperatures (about 
80°) production falls from 44 to 87 per cent. in channel and 
backwaters. It rises, however, 68 per cent. in Phelps Lake, so 
that other causes than temperature may be operative in pro- 
ducing the midsummer decline. The autumnal decline in tem- 
peratures is accompanied by decline in production in the channel 
and in Quiver Lake, but by an increase in other backwaters, 
which exhibit a tendency toward an autumnal pulse. The de- 
cline to winter minimum occurs in December. 
An early spring accelerates, and a late spring retards, the 
vernal pulse, and a late autumn prolongs the autumnal pro- 
duction. Summer heat pulses often attend plankton increases. 
Minimum temperatures are not prohibitive of large plankton 
production. The December production in Phelps Lake in 1898 
(48.14 em.*) exceeds the vernal maximum elsewhere in all local- 
