Factors Determining Vertical Distribution. 419 



FACTORS DETERMINING VERTICAL DISTRIBUTION. 



The following factors contribute to determine the vertical 

 distribution of the limnetic Crustacea. 



1. Food. 



2. Temperature. 



3. Condition of the water in respect to dissolved oxygen and 



other substances. 



4. Light. 



5. Wind. 



6. Gravity. 



7. The age of the members of any given species. 



8. Specific peculiarities. 



Food. 



Food influences the distribution of the Crustacea both by its 

 amount and its quality. As a general proposition, the Crus- 

 tacea should be most numerous where food is most abundant and 

 least numerous where food is least plentiful. Since, therefore, 

 the reproduction of the limnetic algae goes on most rapidly in 

 the upper strata of the lake, it is natural that the Crustacea 

 which feed upon these algae should also be most numerous there. 

 Yet this simple relation of food and eater does not at all cover 

 the facts of vertical distribution. The amount of the algae in 

 lake Mendota is in general so great in proportion to the num- 

 ber of Crustacea that the quantity of food is rarely the pre- 

 dominant factor in vertical distribution. In early spring the 

 Crustacea, and especially Cyclops, increase more rapidly than 

 does the food. But after the opening of summer the food 

 appears to be almost always in excess of the Crustacea, 

 and their distribution, therefore, does not follow variations in 

 its distribution. 



For example, it is well known that the limnetic algae appear 

 in what may be called successive waves of development. A sin- 

 gle species rises to a maximum, predominates for a short time, 

 then declines and nearly disappears, and its place is taken by 

 another species. During the period of decline, especially in the 

 case of diatoms, there is a time when the algae are sinking and 



