LAKE DYNAMICS 15 



spawning conditions, snitable shelter, predator relations, and a number 

 of other conditions. 



The most important factors that determine the productivity of a 

 lake are the elements of fertility, various forms of nitrogen, phosphorus, 

 calcium, and many other elements. These are conditioned by other 

 factors necessary for life such as depth, temperature, bottom types, and 

 dissolved gases. 



Food production is usually considered first in the dynamics of a lake. 

 Plant life is the basis for most fish foods and so green plants are the 

 first stage in the cycle from chemical elements of fertility to fish pro- 

 duction. These plants include not only the large water weeds along the 

 margins, but also the myriads of tiny and often invisible algae that 

 swarm throughout the water. Although only a few species of fishes eat 

 large quantities of plants directly, plants form the food of many of 

 the lower animals eaten by the forage fishes, which are in turn eaten 

 by the game fishes. 



Plants obtain food from the bottom and from chemical substances 

 dissolved in the water. The fertility of a lake therefore starts with the 

 chemical condition of the bottom and the chemical substances dis- 

 solved in the water. Since the water dissolves from the bottom almost 

 everything that is soluble, it is possible to estimate the chemical fer- 

 tility of the bottom by an analysis of the water. 



In a body of water the elements of fertihty necessary for the maxi- 

 mum production of food and fishes are various forms of nitrogen, 

 phosphorus, calcium, and some other substances — the same in general 

 as the elements of fertility in the soil (Surber and Olson, 1937) . Pure 

 water, that is, water with few chemicals dissolved in it, is usually al- 

 most sterile for both food and fish production. On the other hand it is 

 possible to have too much of these substances in the water, which is 

 then called mineral water. Several lakes in western Minnesota, par- 

 ticularly since they were reduced in volume in past drought years, have 

 assumed the character of mineral lakes. 



Carbonates, usually found in some form of calcium carbonate, or 

 lime, are among the important chemical substances present in lake 

 water. When a considerable amount of lime is present, the water is 

 usually called hard, and if the amount is small it is called soft. Lake 

 Superior, the border lakes, and the lakes in the northeastern portion 

 of Minnesota, where limestone is scarce or absent, usually contain soft 

 water. The waters of lakes of this type produce only a small amount 

 of vegetation and fish food. 



Lime and other carbonates in the water are necessary for proper 

 plant growth, because from certain forms of hme (bicarbonates) plants 

 can obtain the carbon dioxide necessary for the manufacture of plant 

 food and the liberation of the valuable oxygen. Carbonates are less 



