281 
tions were sufficiently frequent to trace the movement in plank- 
ton production—shows that this season of the year is wont to 
be a period of rapid change in plankton content. Thus, in the 
river in 1896 on May 13-18 the plankton fell from 3.56 to .S6, or 76 
per cent., in stable hydrographic conditions. A similar phenome- 
non may be involved in the fluctuations in plankton content 
found in this transit of the river. The time intervening between 
the first and last collections was a little over two days. T’o this 
must be added the consideration that the collections represent 
a strip more than 200 miles in length, since we were traveling 
against the current, and, furthermore, that we have to deal 
with the volumetric changes in plankton content, as it passes 
down stream, due to growth and decay. 
Allof these influences are apparently but slight in comparison 
with the effect of certain environmental factors which are local- 
ly dominant within certain sections of the river. Wecandistin- 
guish on the days of collection four sections or minor units of en- 
vironment dominated by different factors. The frst three col- 
lections made in the lower river lie in a region of comparatively 
clear water free from flood invasion. Unfortunately the third 
collection was lost, but the remaining two exhibit a departure 
in the case of the estimated plankton of +12 percent. and of +44 
per cent. in the total catches. The next six collections, covering 
a stretch of 60 miles, from Florence to Browning, were all taken 
in a section of the river invaded by flood water of recent origin 
and poor in plankton, as was evident from the increased tur- 
bidity, the large amount of drift floating, and the discharge 
from tributary streams—principally on the right bank. In such 
conditions the amount of plankton (estimated) is small, and its 
variations form proportionately large percentages of its mean, 
the average departure being +51 per cent., with a range from 
—79to +127—a total of 206 per cent. If, however, simply the 
total catch is taken, the average departure is+5 per cent., witha 
range of —18to +8—a total of 26 percent. In view of theextent 
of the river included in this section—60 miles—and the uneven 
distribution of the flood contributions, it is not surprising that 
