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 
