68 



AQUATIC CONDITIONS 



in late summer or early autumn. Smaller bodies of water are similar 

 in this respect. 



I. LAW GOVERNING QUANTITY (47) 



Liebig's Law of Minimum, as applied to plants, is stated as follows: 

 "A plant requires a certain number of foodstuffs if it is to continue to 

 live and grow, and each of these food substances must be present in a 

 certain proportion. If one of them is absent, the plant will die; if one 

 is present in a minimal proportion, the growth will also be minimal. 

 This will be the case no matter how abundant the other foodstuffs may 

 be. Thus the growth of a plant is dependent upon the amount of the 

 foodstuff which is presented to it in minimal quantity" (47, p. 234). 

 The amount of plankton is determined by the same law. All food sub- 

 stances must be present in the correct proportions. The amount of 

 plankton may be determined by one substance which is deficient in 

 amount. 



2. AGE AND QUANTITY (6 and citations) 



In bodies of water with small outlet, the quantity of plant and animal 

 life probably increases with the age of the water body. This is because 

 the foodstuffs are washed in by the inflowing water, and because rooted 

 plants absorb food from the soil in which they grow, and when they die 

 and decay these foodstuffs are added to the water. Accordingly, the 

 older the pond and the longer rooted vegetation has grown, the greater 

 the quantity of life. This principle is illustrated by an age-series of 

 ponds at the south end of Lake Michigan to be discussed in detail later. 

 The numbers used indicate relative age. Ponds i, 5, 7, 14, 30, 52, 89, 

 and 95 were studied, but especially i, 5, 7, and 14 (6). Tables VI- VIII 

 give a summary of the results. 



TABLE VI 



SHOWING QUANTITATIVE RESULTS OF EXAMINATION OF FACTORS RELATED TO 

 QUANTITY OF PLANKTON 



Average of collections, April, May, June, July, taken over sandy bottom (pond i) or at the top 

 of submerged vegetation (ponds 7 and 14). 



