NET PLANKTON OF LAKE MENDOTA 55 



In some samples of marine net plankton which were obtained during 

 the winter months when diatoms were abundant, Hensen^ found that 

 the ash constituted from 52.1 per cent to 54.7 per cent of the dry ma- 

 terial. The ash of Ceratium amounted to 3.9 per cent of the dry weight ; 

 one sample of copepods contained only 0.45 per cent of ash and another 

 consisting of Calanus yielded 3.78 per cent. Salpa runcinata gave 14.6 

 per cent of ash. 



Brandt^ gives results for the chemical analyses of nine samples of 

 marine net plankton obtained in Kiel bay between September 21, 1892, 

 and September 28, 1893. His data are given in table 51 (p. 218) 

 in the series marked II to X. The nitrogen in these samples varied 

 from a minimum of 1.8 per cent to a maximum of 5.6 per cent of the 

 dry material ; thus, the crude protein ranged from 11.2 per cent to 35.0 

 per cent, representing a little more than a threefold variation in per- 

 centage. This marine material yielded from 1.5 per cent to 8.7 per 

 cent of ether extract and from 8.5 per cent to 61.4 per cent of ash ; the 

 ash contained from 4.5 per cent to 51.2 per cent of silica. 



In the net samples from Lake Mendota the minimum percentage of 

 nitrogen was 3.9 per cent of the dry sample (table 43) and the maxi- 

 mum was 9.9 per cent. In terms of crude protein these results repre- 

 sent 24.5 per cent and 62.3 per cent respectively. The minimum for 

 nitrogen and crude protein, therefore, is more than twice as large in the 

 Mendota material as in the marine catches recorded by Brandt, while 

 the maximum in the former is nearly twice as large as that in the latter. 

 The ether extract in the net samples from Lake Mendota ranged from 

 2.7 per cent to 20.0 per cent ; both of these percentages are larger than 

 the minimum and maximum given by Brandt. The minimum and maxi- 

 mum percentages of both ash and silica were larger in the marine ma- 

 terial than in the Mendota net samples. 



On the basis of these chemical analyses, then, the net plankton of 

 Lake Mendota represents a better class of food material than the ma- 

 rine net plankton, in so far as protein and fat are concerned, because 

 the former contains a larger percentage of these two excellent food 

 substances ; in addition also, the percentage of ash is not as large in the 

 fresh-water as in the marine net plankton. 



Organisms Responsible for Periodic Increase 



During the five years of these obesrvations the vernal increase in 

 the organic matter of the net plankton was due to a rise in the number 

 of diatoms. In three of these years all of the limnetic forms, namely, 

 Melosira, Tabellaria, Fragilaria, and Asterionella, showed a distinct 



^ Fiinf. Ber. Kom. z. wissen. Untersuch. d. d. Meere, 1887, pp. 1-107. 

 ^Wissensch. Meeresuntersucli., Abt. Kiel, N. F., Bd. 3, 1898, pp. 45-90. 



