PHYSICAL AND CHEMICAL FACTORS IX THE METABOLISM OF LAKES 



11 



tion. In this connection the topography 

 of the surroundings and the slun-e develop- 

 ment (ratio of actual shore length to mini- 

 mum perimeter for tlie lake area) are com- 

 plicating factors. With increase in area 

 the enrichening effect of the inshore region 

 is diminished proportionately. Not only 

 are the shore flora and fauna reduced by 

 greater exposure and related factors but 

 the proportion of shore length to total area 

 is rapidly reduced. Finally, the alloch- 

 thonous materials rich in potential nutri- 

 ment, are more thinly spread, especially in 

 the open water regions of large lakes. 



AYith area as with depth the effect on life 

 in the lake is produced through a number 

 of secondary and variable factors. Since 

 one could hardly hope to recognize the 

 specific effects of these factors, the writer 

 (Rawsoii 1930) was led to attempt a demon- 

 stration of the total effect of depth and 

 area on the amount of bottom living animals 

 in various lakes. The accompanying graph 

 (Fig. 2) was used to illustrate the result. 



iJO 



UAKE MEHOOTA 



1000 2000 3000 4000 



PaODUCT OF AREA (uPT0 4OS<3MILE5)XDEPTH (uPTO IOOFT) 



Fig. 2. The quaiititv of bottom animals as related 

 to deptli and area in lakes. 



It will be noted that the average weight of 

 bottom organisms per unit area is plotted 

 against the product of depth and area, the 

 latter with certain restrictions. It was be- 

 lieved that the effects of increase w^ere not 

 marked after area reached 40 square miles 

 and depth 100 feet. The whole procedure 

 is rough and the demonstration more cpiali- 

 tative than quantitative. However, the 



curve does suggest a correlation between 

 density of bottom population and the prod- 

 uct of depth and area. It is also of interest 

 that lakes of a clearly eutrophic nature com- 

 pose the upper arm of the curve which lies 

 at a considerable angle to the lower arm on 

 which are oligotrophic lakes. Possibly our 

 estimate of the upper limit of the depth 

 effect (100 feet or 32 meters) is too high 

 and the data should be recalculated using 

 Thienemann's value of 18 meters. 



The shape or contour of the bottom ap- 

 pears to affect the life in a lake in a way to 

 some extent independent of the above men- 

 tioned eff'ects of depth and area. It is 

 clear that a lake with gradually sloping 

 sides will be somewhat richer than one with 

 a steep decline into deep water. This dif- 

 ference between what has been called V- 

 shaped and U-shaped contours may be ex- 

 pressed in various ways. It is commonly 

 indicated by calculating the relative areas 

 of the ^-arious depth zones or by construct- 

 ing a bathymetric curve. Possibly a factor 

 "littoral development" indicated by ex- 

 pressing the area of the littoral zone as a 

 percentage of the total area would be a 

 justifiable innovation, for it appears that 

 the rich flora and fauna of this area are 

 important factors in the production of a 

 lake. 



Varying opinions have been expressed as 

 to the significance of the gradient of the 

 lake bottom. Alsterberg (1930) points out 

 the importance of greater or lesser areas of 

 contact between the water and the bottom 

 surface. He even goes so far as to consider 

 this the chief factor in determining oligo- 

 trophy and eutrophy. Strpm (1930) con- 

 siders the littoral development of special 

 significance in modifying the trophic con- 

 dition of a lake. Undoubtedly the U-shaped 

 basin with steep sides is in harmony with 

 other oligotrophic influences and strong lit- 

 toral development with eutrophy. The 

 question is to some extent related to that of 

 mean depth since in lakes of the same area 

 that with the greater depth will tend to 

 have steeper shores. This should be kept 

 in mind when considering Alsterberg 's 

 (1929, 1930) rather violent criticism of 



