48 FRESH-WATER BIOLOGY 



71.36 cc. per cubic meter the maximum; 684 cc. per cubic meter 

 (Turkey Lake, Ind.) is the largest amount recorded by Juday. 



Small streams and lakes with large inflow and outflow have little 

 plankton. Large amount of plankton is usually associated with 

 much CO2, little oxygen, and a large amount of dissolved carbonate. 



The amount of plankton fluctuates from season to season. The 

 maximum for the Illinois River is from April to June. It gradually 

 decreases until December and January, when the minimum is 

 reached. The light of the moon may increase photosynthesis and 

 thus the amount of phyto-plankton (Kofoid). The maximum of 

 Entomostraca was found by Marsh to fall in July, August, and 

 September, differing in different years. In small bodies of water an 

 abundance of plankton is usually, though not invariably, associated 

 with a large quantity of larger animals and rooted plants. Large 

 lakes like the Great Lakes are exceptions to this because of the 

 absence of shallow water vegetation. 



Liebig's Law of Minimum has been applied to plankton by 

 Johnstone who states it as follows: "A plant requires a certain 

 number of foodstuffs if it is to continue 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 that 

 foodstuff which is presented to it in minimal quantity." The 

 amount of plankton probably follows the same law. All food 

 substances must be present in correct proportions. The amount 

 of plankton may be determined by a deficiency in the amount of 

 one substance. 



The quantity of plant and animal life probably increases with 

 the age of bodies of water with small outlet (see Fig. 7, p. 58). 

 This is because foodstuffs are washed in with inflowing water, and 

 because rooted plants absorb food from soil in which they grow, and 

 when they die and decay these foodstuffs are added to the water 

 and made available to plankton and to animals in general. Accord- 

 ingly, the older the pond and the longer rooted vegetation has 

 grown, the greater the quantity of Hfe up to the time the pond 



