COMMUNITIES IN STANDING WATERS 337 



in the lakes of the plains. The color of the water in colder lakes, 

 consequently, is yellowish-green (because of diatoms and Ceratium), 

 in warmer lakes in spring and late fall also yellowish-green; but in 

 summer it is a bluish-green. 



Annual variations are chiefly quantitative. The plankton is less 

 well developed in winter than in summer, but it has certainly not dis- 

 appeared. Many animal forms, in fact, reach their maximum of devel- 

 opment in winter. Thus the ratio of the winter plankton to the summer 

 plankton in Lake Constance is 1:2, in the lakes of north Germany 

 1:20. 33 In Lake Mendota (Wisconsin) the March minimum is almost 

 one-fourth of the April maximum. 30 During the course of the year 

 definite changes in the composition of the plankton arise because of 

 the fact that the breeding season of various plant and animal species 

 occurs at different times. Perennial plankton forms, even though they 

 are not always numerous, are distinguished from periodic forms which 

 occur only at definite seasons of the year. Those without rest periods 

 belong to the perennial group, e.g., most copepods, many Bosmina, and 

 rotifers such as Anuraea cochlearis and Asplanchna priodonta. Exam- 

 ples of periodic forms are the larvae of the triangle mussel Dreissena, 

 and animals which have resting periods, like most of the rotifers, 

 Notholca striata, e.g., found only in the winter, and Cladocera. In 

 equal amounts of plankton from the same lake, the combination of 

 species may be very different at different times. 



The various depths of the open water, moreover, also exhibit a 

 stratification of the plankton, which is determined primarily by the 

 amount of light, the conditions of temperature, and the amount of 

 oxygen, but partly also by the movements of the water. This stratifica- 

 tion varies with day and night and with summer and winter. In Lake 

 Constance, the plankton forms are spread evenly to great depths in 

 the winter, when temperature and lighting are more nearly equalized, 

 while in the summertime the masses of the plankton extend only about 

 30-35 m. in depth, and during the daytime the uppermost layer of 

 about 1 m. in thickness is poor in plankton; within this mass the 

 individual plankton animals, again, stay in the layers most favorable 

 to them (Fig. 99). 34 In the Black Lake (in the Bohemian Forest) in 

 the summertime, Holopediam gibberum (Fig. 79) and Cyclops strenuus 

 are found in the upper 3 m., Daphnia ventricosa and Bosmina bohemica 

 in deeper regions; only Cyclops strenuus is found in larger numbers 

 during later months. 35 In Lake Lucerne the plankton extends to a 

 depth of 65 m. in the summer, in Lake Zug to about 80 m., in Lake 

 Geneva to 100 m.; the lower limit lies much deeper in all these lakes 

 in the wintertime. The graphs of Figs. 99 and 100 show, furthermore, 



