THE PERIODICITY IN THE MULTIPLICATION OF THE ORGANISMS OF 



THE PLANKTON. 



One of the most obvious conclusions brought to light by the 

 detailed study of the volumetric fluctuations of the plankton pub- 

 lished in Part I. of this report, and most strongly reinforced by the 

 statistical data showing the fluctuations in the numbers~of the indi- 

 viduals of the various species and in the sums total of the various 

 biological groups represented in the limnetic fauna and flora, is that 

 plankton production is fundamentally rhythmic or periodic in 

 character, viewed either in its constituent elements or as a whole. 

 This total result is simply the sum of a like phenomenon pervading 

 more or less completely and coincidently the reproductive cycles, 

 the rise and decline in the numbers of the typical constituents of 

 the plankton. The exceptions to this rhythm are usually found in 

 those organisms which are adventitious in the plankton and have 

 their centers of growth and distribution in other regions than the 

 open water. 



Many illustrations of this periodic movement in the multiplica- 

 tion of organisms of the plankton have been cited in the preceding 

 pages and may be seen in the accompanying plates. As an illustra- 

 tion for discussion in detail we may take the pulse of July, 1898, 

 shown in the volumetric data of Table III. and Plate XII. of Part I. 

 The fluctuations in the biological population during this period are 

 also tabulated in Table I. of this paper, and graphically presented 

 in Plates II. and IV., which exhibit the movement in the totals of 

 the Chlorophycecz, Bacillariacece, and chlorophyll-bearing Masti- 

 gophora, and of the Rotifera and Crustacea. 



In the volumetric data the pulse rises from a minimum of .14 cm. 3 

 per m. 3 on July 5 to a maximum of ,88 cm. 3 on the 19th, declining 

 again on the 26th to the second minimum, of .67 cm. 3 Its duration 

 is thus four weeks and its amplitude, in comparison with many 

 other pulses in the records, relatively slight. It occurs in the more 

 stable conditions of declining river levels and midsummer tempera- 

 tures. The following list gives the names of the more or less typical 

 planktonts considered in the discussion of this pulse. Others, 

 largely adventitious or insignificant in numbers, might be added 



(20) 291 



