147 



the seasonal distribution of individual species and these recurrent 

 plant pulses will be discussed in connection with the various species 

 wherever the data are available. For the present it will suffice to call 

 attention to such correlation as exists between fluctuations of the 

 phytoplankton and the total Ploima. The table on the preceding page 

 gives the location and amplitude of the maxima of the ploiman pulses, 

 and a graphic presentation of the seasonal curve of distribution of the 

 total Rotifera will be found in Plates III. and IV. On comparison of 

 the ploiman pulses with those of the chlorophyll-bearing organisms, 

 graphically presented in Plates I. and II., it will be found that 15 of 

 the 33 pulses of Ploima contained within the period covered by the 

 plates coincide in location with the plant pulses ; that 12 follow at the 

 next collection, usually a week later, and 3 within a fortnight ; while 

 only 3 of the 33 exhibit no such correlation. The data suggest 

 strongly the agency of the plant pulses in building up the Ploima, 

 and that the food relations are fundamental in the fluctuations of 

 these planktonts. 



DISCUSSION OF SPECIES OF PLOIMA. 



Anuraza aculeata Ehrbg. Average number, 1,839. In 1898 

 this species has a very well-defined and characteristic seasonal 

 distribution (Table I.). It first appears March 8 at 37, increases 

 to a maximum of 45,200 on May 10 at 61. then declines, and 

 disappears June 14 at 83. The curve of its occurrence in this year 

 is a very symmetrical one. It reappears on December 27 at 32, and 

 there are scattered occurrences through the winter months of 1899. 

 Records in other years suggest in the main a similar distribution. 

 In 1896 it first appeared January 6, rose to a pulse of 6,550 on May 8 

 at 76, and, on the decline of the June rise, there was a second and 

 larger pulse of 29,600 on June 17 at 76. It reappeared on Decem- 

 ber 29, and in 1897 reached a vernal maximum of 22,400 on May 25 

 at 66, then disappeared, and was not again noted in the following 

 winter nor until March 8. In 1894 the last vernal record was made 

 June 12, and on September 4, at 78, there was an autumnal pulse 

 of 13,825 a phenomenon not repeated in subsequent years. The 

 normal course of its seasonal distribution in the river plankton seems 

 to be as follows: reappearance in December when minimum 

 temperatures have been reached; slow multiplication during the 

 winter, and a well-defined pulse on the decline of the spring flood in 



(in 



