Marsh — The Planhton of Fresh Water Lakes. 173 



fauna do not, by any means, behave in tbe same way. They 

 have most decided individual peculiarities, so that we cannot 

 speak of the movements of the fauna as a whole, but each species 

 must be considered by itself. Some of them do not move at all 

 vertically, but have the sam.e distribution from one end of the 

 day to the other. Others, like the larval forms of the copepods, 

 are more numerous at the surface in the daytime than in the 

 night. Some have a very pronounced migration. This is par- 

 ticularly true of Leptodora which is rarely found at the surface 

 in the daytime, but appears at almost exactly forty-five minutes 

 after sunset, remains at the surface during the night, and dis- 

 appears again at just three-quarters of an hour before sunrise. 



Most of the larger Crustacea which form the great body of the 

 plankton do migrate in this way, and it was natural, perhaps, 

 to infer that the whole plankton moved up and down. 



The limits of this vertical mi^rration it is verv difficult if not 

 impossible to fix. Most of the movement is within one meter 

 of the surface, the most marked changes being within one-half 

 meter of the surface, and below three meters the amount of 

 movement is very slight. Eight determining factors have been 

 listed by Professor Birge as controlling the vertical distribution 

 of Crustacea: food, temperature, condition of the water in re- 

 spect to dissolved oxygen and other substances, light, wind, 

 gravity, age and specific peculiarities. Of these factors, by far 

 the most important are food, temperature and light. Inasmuch 

 as the food supply is controlled by temperature and light, we 

 may speak of these two factors as, in the main, controlling tho 

 vertical distribution of the limnetic plankton. Of these two 

 factors, temperature is the most important, although light has 

 a marked effect on many species. In the winter season when the 

 waters of all the lakes are very nearly uniform in their tempera- 

 ture from top to bottom, the vertical distribution of the lim- 

 netic fauna is much more uniform than in summer. 



In the smnm.er season the most marked changes in vertical 

 distribution are correlated with the vertical changes in tempera- 

 ture. This is most distinctly seen in the deeper lakes. In these 

 lakes it is a surface layer of greater or less depth w^hich is 

 warmed, the deeper layers feeling the effect of the summer's 



