14 



PROBLEMS OF LAKE BIOLOGY 



heat income is only significant in comparing 

 lakes of sufficient depth to store the amount 

 of heat climatically possible. This would 

 restrict its use to lakes of considerable size 

 with mean depths of over 30 meters and in 

 many regions such lakes are rare or absent. 

 It would be more just to say that heat bud- 

 gets may be more readily compared and 

 interpreted in lakes of more than 30 meters 

 depth but they may be and have been ap- 

 plied to shallower lakes with useful results. 

 It would seem, however, that in the basic 

 question of productivity we are less con- 

 cerned with the total heat acquired than 

 with the temperature of the trophogenic 

 region (chiefly the epilimnion) and the 

 length of the growing season. 



Thermal stratification has been indicated 

 in the foregoing chart of interrelations, as 

 primarily dependent on depth, wind, and 

 insolation. It is also shown as affecting 

 the penetration and utilization of oxygen, 

 thus occupying a central position in the 

 complex of factors which control the in- 

 ternal economy of the lake. Although 

 thermal stratification is one of the most 

 familiar and important of the physical phe- 

 nomena in lakes, it has still some features 

 which will bear explanation. The warm- 

 ing of the hypolimnion during the summer 

 has been considered by Alsterberg (1930) 

 and Richer (1937) with the conclusion that 

 in some very clear lakes direct solar radia- 

 tion must be responsible. As Ricker points 

 out, although the hypolimnion of Cultus 

 lake acquires most of its heat by direct 

 radiation, it is still necessary to assume 

 some mixing (turbulence) to account for 

 its distribution. Karsinkin et al (1930) 

 demonstrated that in Lake Glubokoje tem- 

 perature and oxygen were stratified but not 

 horizontally. Thus turbulence of the hori- 

 zontal kind proposed by Alsterberg could 

 not have existed. Whitney (1938) in re- 

 cent studies of transparency in the waters 

 of Wisconsin lakes has shown extensive and 

 irregular microstratification in the thermo- 

 cline and hypolimnion. The extent of this 

 microstratification would suggest that there 

 is no extensive turbulence in these lakes. 



Recently Hutchinson (unpublished note) 



has considered the problem of turbulent 

 conduction of heat using the mathematical 

 treatment suggested by McEwen (1931). 

 Analyzing the warming of the hypolinniion 

 in Lake Mendota and Linsley pond, where 

 radiation was known to be negligible, he 

 concludes that in the upj^er part of the 

 hypolimnion (9 to 15 meters in Mendota) 

 warming is accounted for satisfactorily by 

 the assumption of a constant turbulence in 

 this region. In the lower part of the hypo- 

 limnion warming proceeds more rapidly 

 than would be expected. He considers it 

 very unlikely that turbulence could be in- 

 creased in the lower hypolimnion and sug- 

 gests that the condition is probably the re- 

 sult of currents near the bottom of the lake 

 bringing bicarbonate, etc., into the water. 

 These currents might not run quite horizon- 

 tally but would tend to descend slightly 

 along the bank to enter a layer of the same 

 density but of lower temperature. This 

 would provide a means of heat transport 

 other than turbident conduction. He sug- 

 gests also that a horizontal current of a few 

 meters per day might account for the above 

 mentioned microstratification effects of 

 Whitney, also for his own observations on 

 alkalinity, and still not be greatly distorted 

 by turbulence of the order which he has 

 calculated for these lakes. 



A further observation of great interest 

 has been provided by Welch and Eggieton 

 (1932, 1935) in their work on submerged 

 depressions in Douglas Lake. The tem- 

 perature curves for seven such depressions 

 showed remarkable differences in stratifica- 

 tion in dift'erent parts of the same lake. 



The direct influence of higher and lower 

 temperature on metabolism is involved in 

 the primary division of eutrophic and oligo- 

 trophic lakes. The rapid production in an 

 eutrophic lake is partly dependent on the 

 higher temperatures of its epilimnion. On 

 the other hand, in extreme oligotrophy, i.e., 

 the panoligotrophy of Pesta (1929), the low 

 temperature is a major if not the limiting 

 factor in production. Lundbeck (1934) 

 thinks it is limiting and points to the ex- 

 tremely oligotrophic lakes of the Alps as 

 examples of thermal oligotrophy. Hutch- 



