33 



thus predominant only during the warmer part of the year; and 

 while autumnal and vernal pulses occur, there is no sustained mid- 

 summer minimum intervening between them. The pulses in 

 Scenedesmus as a rule follow the volumetric pulses as shown in silk- 

 net catches (Pt. I., PI. XI. and XII.). Thus in 1897, on September 

 14 and 21, the plankton measures 19.8 and 3.0 cm. 3 per m. 3 , respec- 

 tively, Scenedesmus quadricauda numbering 20,700,000 and 151,- 

 200,000; and, again, on October 5 and 19 the plankton measures 

 12.92 and 1.86 cm. 3 , and this alga numbers 93,600,000 and 154,- 

 800,000. Its share in the volumetric pulses is thus indirect to a 

 large degree, and is perhaps modified by food relations. 



Schroederia setigera (Schroder) Lemm.* Average number, 

 21,450,000. In 1897, 69,040,912. It appears in all months of the 

 year and in almost every collection. It has well-defined vernal and 

 autumnal pulses separated by the summer period, in which only 

 minor pulses occur. In 1898 midwinter numbers are as high as 

 those of midsummer. Schroederia is thus truly a perennial plank- 

 tont. The vernal pulse appears in 1897 on April 27 at 302,400,000, 

 and in 1898 on May 3 at 150,000,000. The autumnal pulse in 1897 

 culminates on September 21 at 565,200,000, and is followed by sec- 

 ondary culminations on October 26 at 136,800,000, and on Novem- 

 ber 23 at 203,400,000. In 1898, when hydrographic conditions 

 were less stable, the autumnal pulse reached only 50,400,000, on 

 September 6. This is followed by minor pulses, declining to a mini- 

 mum in the following February. It disappeared in the collections 

 with the flood waters of March, 1899. The sequence of these sec- 

 ondary pulses follows much the same course as has been described 

 for other species, namely, maxima at intervals of approximately 

 a month (two to six weeks) separated by more or less sharply 

 defined minima. There are twelve such pulses (including the major 

 ones) in 1898 and an interval of seven weeks in March- April in 

 which none occurs. Six pulses appear in the last five months of 

 1897. 



The optimum temperatures as indicated by the position of the 

 vernal (60 in both 1897 and 1898, as shown in Table III., Pt. I.) 

 and autumnal (71 in 1897 and 79 in 1898) pulses lie between 60 

 and 80. This appearance of the vernal pulse at a lower temperature 

 than the autumnal (usually about 10 lower) is not confined to this 

 species but is a general phenomenon among other Chlorophycea. 



