12 



PROBLEMS OF LAKE BIOLOGY 



Thienemaiin's conclusion as to mean depth. 

 The whole question is dealt with thoroughly 

 by Grote (1934. 1934a). We should at 

 least be safe in saying that V-shaped basins 

 and low mean depths are conducive to 

 eutrophy. 



Irregularities of bottom conformation 

 have been shown by Welch and Eggleton 

 (1932, 1935) to cause a degree of isolation 

 of submerged depressions within the main 

 basin. The effect of such irregularities on 

 deep currents is little known but must also 

 be of some significance. 



THE GEOLOGICAL NATURE OP THE 

 SURROUNDINGS 



It is self-evident that the nature of the 

 drainage area must in a large measure de- 

 termine the kinds and amounts of primary 

 nutritive materials in a lake. The composi- 

 tion, age and weathering of the rock, the 

 soil and vegetation cover, the extent of the 

 drainage area and the amount of run-off 

 are all important considerations. It is per- 

 haps less evident that nutritive materials 

 washed in in this way are not the only de- 

 termining factors in the biological produc- 

 tivity of the lake. As we have pointed out 

 in the foregoing section, the shape of the 

 lake basin, especially its depth, and the cli- 

 matic factors affecting water circulation are 

 potent factors in this connection. 



The situation was analyzed by Str0m 

 (1930) who saw "two great conflicting ten- 

 dencies: the tendency of each lake to form 

 a microcosm where the evolution of its life 

 processes are determined by the morphol- 

 ogy of its basin; and the tendency that 

 works towards making the lakes mere prod- 

 ucts of their substrates and drainage areas. " 

 The same problem was considered by Lund- 

 beck (1934) in his distinction of primary 

 or edaphic oligotrophy from secondary or 

 morphometric oligotrophy, and quite re- 

 cently it has been further discussed by 

 Hutchinson (1938). 



The emphasis which has been placed on 

 the conflict of these influences may have 

 obscured the fact that while they are largely 

 independent they are not necessarily con- 

 flicting. The extreme cases of eutrophy 



and oligotrophy are usually those in which 

 both edaphic and morphological influences 

 have worked in harmony. When these in- 

 fluences are not in harmony there is often 

 a striking development of eutrophy in spite 

 of surroundings which would normally im- 

 pose oligotrophy. To cite a single example. 

 Mountain Lake, Virginia, studied by Hut- 

 chinson and Pickford (1932), shows a con- 

 siderable tendency toward eutrophy in spite 

 of a small drainage area and water contain- 

 ing extremely small amounts of salts, nutri- 

 tive and otherwise. 



The question as to which influence is the 

 more important seems to us much like the 

 old question of heredity and environment. 

 The edaphic influences determine the origi- 

 nal endowment and the morphology con- 

 trols later development. Which is the more 

 active can only be decided for particular 

 lakes or regions and there only incom- 

 pletely. Str0m (1930) suggests that Nau- 

 mann working on lakes in south Sweden 

 was naturally led to overemphasize the im- 

 portance of edaphic influence while Thiene- 

 mann working on the distribution of dis- 

 solved oxygen was led to exaggerate the 

 importance of morphology. It may be 

 added that Str0m himself has made rather 

 complete geological studies in relation to the 

 lakes which he has examined and is, there- 

 fore, expressing a well founded opinion. 



Our conclusion is that while the edaphic 

 factors determine the kinds and amounts 

 of primary nutritive materials the morphol- 

 ogy of the basin and the climate may to a 

 large extent determine the utilization of 

 these materials. 



TEMPERATURE 



The essentials of the thermal cj'cle in 

 temperate lakes have long been common 

 knowledge and one speaks glibly of vernal 

 and autumnal periods of circulation and 

 summer and winter stagnation. In inten- 

 sive work we recpiire a more complete defi- 

 nition of these periods such as has been pro- 

 vided in the discussion of Yoshimura (1935) 

 and further revised by Ricker (1937). In 

 these analyses the four primary periods are 

 subdivided into their complete and partial 



