NET PLANKTON OF LAKE MEN DOT A 57 



sphaerium, and Daphnia hyalina. The December peak of 1913 was 

 correlated with a rise in Fragilaria. 



In 1915 the peak noted in October was correlated with a rise in Fragi- 

 laria, while the increase in organic matter during November and De- 

 cember corresponded to a gradual increase in the numbers of Tabellaria 

 and Stephanodiscus. Asterionella and Stephanodiseus were respon- 

 sible for the October peak in the curve for 1916. 



The summer and winter minima correspond to relatively small num- 

 bers of the various organism's in the net plankton. In late spring or 

 early summer the diatoms decline to a minimum number and the green 

 and blue-green algae then become the predominant phytoplanktonts. 

 While the light and temperature conditions are favorable for these 

 forms during the summer, yet they do not develop in sufficient abun- 

 dance in Lake Mendota to produce a maximum comparable to the max- 

 ima of the diatoms in the spring and in the autumn. In winter both tem- 

 perature and light conditions are unfavorable for the growth of the 

 phytoplankton ; the latter is especially unfavorable after the lake be- 

 comes covered with ice and snow. As a result the numbers of these 

 organisms decline as the winter season advances. 



The numerical results obtained in this investigation show more or 

 less pronounced maxima of Crustacea and rotifers in spring and autumn 

 and Birge ® also noted these two periods of maxima, with an additional 

 one in July, in his studies on the Crustacea of Lake Mendota. Just how 

 prominent a part these two groups of organisms play in increasing the 

 quantity of organic matter in the net plankton is difficult to determine. 

 When allowed to settle, the net samples taken during the spring and 

 autumn maxima show a distinct preponderance of algal material ; con- 

 sequently these maxima have been attributed chiefly to the latter forms. 

 Thus, the large growths of phytoplankton at such times tend to mask 

 the increases shown by Crustacea and rotifers. 



Numerically, of course, the phytoplanktonts are more abundant, but 

 a comparatively small increase in the number of the larger Crustacea is 

 equivalent to a very marked increase in the algae so far as weight is 

 concerned. Several determinations show that it takes 225 individuals 

 of Cyclops of mixed sizes to yield one milligram of dry material ; of the 

 other plankton Crustacea it takes 135 Diaptomi, 140 Daphnia retro- 

 curva, and 75 Daphnia hyalina to yield this'amount of dry material. 

 Among the rotifers it takes about 125 colonies averaging one hundred 

 individuals each, or 12,500 individuals of Conochilus volvox, to yield 

 one milligram of dry material, while it requires only about one-tenth 

 as many, or 1,250, large specimens of Asplanchna for this amount. 

 About 650 colonies of Volvox of mixed sizes yield a milligram of dry 



• Trans. Wis. Acad. Sci., Arts, and Let., Vol. XI, 1898, pp. 274-448. 



