SOME PHYSICAL AND CHEMICAL FACTORS IN 

 THE METABOLISM OF LAKES 



By D. S. RAWSON 



UNIVERSITY OF SASKATCHEWAN, SASKATOON. SASK., CANADA 



Introduction 



The lake with its plant and animal in- 

 habitants ]n'esents a snffieiently unified bio- 

 type that it may, with certain reservations, 

 be termed a microcosm. If we consider it 

 as a liyin<>' unit we may also characterize its 

 complex internal exchang-es as metabolism. 

 The biota in question and its activities are 

 to a laro'e extent controlled by physical and 

 chemical influences both internal and ex- 

 ternal to the lake. It is thus desirable that 

 we should beo'in with a consideration of 

 these primary physical and chemical 

 factors. 



In reviewin<i' so broad a field it has been 

 necessary to select factors which are con- 

 sidered more fundamental and the em- 

 phasis is on their interrelations rather than 

 specific effects. The subject of lisiht and 

 the direct influence of various factors on 

 org'anisms in the lake will be considered in 

 other papers of the series. 



The Interrelation of Factors 



In approaching the question of physical 

 and chemical factors influencing the me- 

 tabolism of a lake we might ask certain sig- 

 nificant questions. AVhat are these factors? 

 Which of them are most important? Can 

 their separate effects be recognized? How^ 

 are they interrelated? 



In answer to the first question, there are 

 many factors and listing them would serve 

 no useful purpose at the moment. Their 

 relative imi^ortance and separate effects will 

 be demonstrated in some measure in our 

 later discussions. With respect to the last 

 question the factors are interrelated in very 

 many ways. The logical relations are so 

 complex that we have been led to construct 

 the accompanying chart (Fig. 1), as a 

 graphic aid to our discussion. 



Withont considering the chart in detail 



you will note in a general way how, starting 

 with geographic location, we have indicated 

 various chains and complexes of factors 

 which have a part in determining the 

 trophic nature and biological productivity 

 of the lake. While the scheme is elaborate 

 it is still not complete. No attempt has 

 been made to indicate the relative impor- 

 tance of i^articular factors, although some- 

 thing of the sort might be possible with 

 respect to individual lakes rather than for 

 lakes in general. Possibly each investigator 

 should construct his own chart. In any 

 case we think it desirable to visualize some- 

 thing of the maze of relations before enter- 

 ing on the separate discussion of factors as 

 we shall now proceed to do. 



the morphology of the lake basin 



The effects of the physical dimensions of 

 a lake on its population and metabolism is 

 a matter of common observation. The 

 fundamental nature of this relation was 

 expressed by Thienemann in 1927 when he 

 demonstrated that the nature of the lake 

 basin and especially its mean depth, was a. 

 most important factor in determining lake 

 type. It should be noted, however, that 

 the effects of morphology are often indirect, 

 involving secondary factors such as tem- 

 perature, oxygen, and the production of 

 nitrogenous materials. The effects may 

 also be greatly modified by the geological 

 nature of the surroundings. 



Tlie mean de]:)th of a lake is probably, as 

 Thienemann believes, its most significant 

 dimension. After examining data from 

 many lakes he concluded that most eu- 

 trophic lakes had mean depths of less than 

 18 meters and oligotrophic more; also that 

 the volume of the epilimnion was greater 

 than the hypolimnion in eutrophic and 

 smaller in oligotroi)hic lakes. In spite of 



