PHYSICAL AMD CHEMICAL FACTORS IN THE METABOLISM OF LAKES 



15 



iiisoii (1938) does not agree believing that 

 the condition in tliese lakes is due to chemi- 

 cal factors rather than temperature. 



In the foregoing pages we have dealt 

 briefly with what we consider to be the chief 

 physical factors in the metabolism of lakes, 

 excepting of course the extremely import- 

 ant factor of light. AVe may now turn our 

 attention to certain outstanding chemical 

 factors beginning with the question of dis- 

 solved gases. 



DISSOLVED OXYGEN 



In temperate lakes the supply of oxygen 

 is largely renewed during the vernal circula- 

 tion. When thermal stratification appears 

 the amount of oxygen in the liyiiolimnion 

 begins to decrease and as stagnation ad- 

 vances it may disappear from the whole of 

 this region. In the thermocline there may 

 be a sharp increase or decrease in dissolved 

 oxygen and in the epilimnion it remains 

 relatively high all summer. "With the 

 autumn turnover and circulation the oxy- 

 gen of the deep water is restored, although 

 it may be depleted once more in a winter 

 stagnation period under the ice. Among 

 these changes in the distribution of oxygen, 

 we find the summer depletion in the liypo- 

 limnion of greatest significance in the me- 

 tabolic activities of the lake. 



Thienemann (1928) developed a number 

 of standards or values for the comparison 

 of amounts of oxygen in lake waters. Vari- 

 ous investigators took up the problem and 

 a long series of papers has appeared, many 

 of them controversial. The result has been 

 an improvement in the treatment of the 

 physical data and the demonstration that 

 certain of the early standards or indices 

 were of little use. Thus Thienemann 's ex- 



O IT 



pression ^^-^ indicating the ratio of the 



total oxygen of the liypolimnion to that of 

 the epilimnion is found somewhat unsatis- 

 factory. It may give misleading results 

 especially in oligotrophie lakes with small 

 hypolimnia. It would seem also that the 

 oxygen deficit of the epilimnion or any in- 

 dex involving this calculation is of little 

 significance as long as the amount of oxv- 



gen released by photosynthesis remains an 

 unknown (piantity. Thus the hypolimnial 

 deficit is the more widely used. 



Alsterberg (1930) improved the expres- 

 sion of oxygen deficits when he calculated 

 his so-called "absolute deficit." Maucha 

 (1931) and Richer (1934) carried the con- 

 siderations still further. Strpm (1931) 

 considered that the deficit should be ex- 

 pressed per unit area of the liypolimnion in 

 oligotrophie lakes and per unit area of lake 

 in eutrophic. Grote (1934, 1934a, 1936) 

 has considered the problem minutely and 

 has shown the deficit to be of very complex 

 origin. Hutchinson (1938) agrees with 

 Strpm as to the necessity for relating de- 

 ficits to area of the hypolimnion. In the 

 case of lakes where the oxygen of the hypo- 

 limnion is completely exhausted he suggests 

 that the "real" deficit can be obtained by 

 adding to the apparent deficit the amount 

 of oxygen necessary to oxidize the existing 

 quantities of such substances as methane 

 and hydrogen sulphide, substances which 

 would not have formed in the presence of 

 abundant oxygen. This procedure seems 

 to be logical but the data are not often suffi- 

 ciently complete to make it practical. It is 

 also a question whether this correction goes 

 far enough, for there must be dead plank- 

 ton, bottom detritus, and other materials 

 which would have been decomposed using 

 additional amounts of oxygen had they 

 been available. A noteworthy feature of 

 Hutchinson's calculations is the expression 

 of hypolimnial deficits per day, recognizing 

 the desirability of including the time factor 

 and dealing with rates rather than standing 

 quantities. 



The interest in expression of hypolimnial 

 oxygen deficits arises from the possibility 

 of their use as a measure of lake produc- 

 tivity. The basic assumption was that 

 much of the organic material produced in 

 the trophogenic (photosynthetic) region of 

 a lake sinks into the deeper water, the tro- 

 pholytic or decomposition zone where it is 

 oxidized. Tliis assumption has been seri- 

 ously questioned. Alsterberg (1927) pro- 

 posed that the chief consumption of oxygen 

 was in tlie bottom deposits and that the 



