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 .86, 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. To 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. 



All of 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. We can distin- 

 guish 0)1 the da lis of coUecfion four sections or minor units of en- 

 vironment dominated by different factors. The first 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 w^ater 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 

 —79 to +127— a total of 206 per cent. If, however, simply the 

 total catch is taken, the average departure is ±5 per cent., with a 

 range of —18 to +8 — a total of 26 per cent. In view of the extent 

 of the river included in this section — 60 miles — and the uneven 

 distribution of the flood contributions, it is not surprising that 



