53 



This Melosira is a perennial planktont in that it occurs in every 

 month of the year in the river. Its appearances from December 

 to March are, however, irregular, and its numbers small. Its large 

 pulses above 1,000,000 all lie between May 15 and October 1, 

 with the single exception of the pulse of April 24, 1896, culminating 

 at 2,056,400, in temperatures of 72, occurring fully a fortnight 

 earlier than usual. The major pulse seems normally to occur in 

 June; at least in 1896 and 1898, when collections were frequent at 

 this season of the year, such pulses appear on the llth at 12,940,000 

 and on the 21st at 32,114,880. A June pulse also appears in 1895. 

 September pulses appear in 1895, on the 12th, at 2,254,182, and in 

 1898, on the 27th, at 5,499,840. There is, however, no well-defined 

 vernal and autumnal growth period, since large pulses occur through- 

 out the whole summer. The greatest pulse on record (111,456,000) 

 is on July 21, 1897, and in 1898 there are three minor pulses between 

 those of June and September. Including the major pulses, there 

 are in 1895 five, in 1896 six, in 1897 five, and in 1898 eight, pulses 

 at intervals of two to six weeks between May and October, the ones 

 at either end of the season being often but slightly developed, the 

 remainder usually running from 1,000,000 to 5,000,000. 



This species is predominantly a summer planktont, and its 

 optimum temperature lies above 70, the greatest number recorded 

 appearing at 81. This is one of the most abundant diatoms of 

 the potamoplankton, and in our waters it attains its greatest de- 

 velopment during the season of the minimum occurrence of nitrates, 

 in whose utilization it is quantitatively an important agent. It 

 fills the gap between the vernal and autumnal or hiemal appear- 

 ances of Asterionella and Fragilaria, thus providing a continuous 

 source of food for the zooplankton with w r hich it is associated. It 

 is, by virtue of its numbers, its size, and its seasonal distribution, 

 quantitatively and ecologically the most important of all the 

 diatoms of the plankton of the Illinois River. 



The only factor in the environment to which the limitation of the 

 rapid growth of this species 'to the May-October period can be re- 

 ferred is temperature. There are but three instances in the records 

 of Melosira exceeding 100,000 per m. 3 at temperatures below 60, and 

 one of these is but a few days prior to the attainment of that temper- 

 ature. It cannot be food which deters its development below this 

 point, since the nitrates at least are then most abundant (Pt. I., PL 



