16 



PROBLEMS OF LAKE BIOLOGY 



reduction throughout the hypolimnion was 

 produced by contact with the bottom layers 

 through the agency of hypolimnial water 

 movements, i.e., turbulence. It has also 

 been demonstrated by Rossolimo and Kus- 

 nezowa (1934) that in some lakes the bac- 

 terial oxidation of gases such as methane 

 and hydrogen may be responsible for much 

 of the oxygen reduction. These gases were 

 produced by decomposition of the bottom 

 ooze and passed upward into the hypolim- 

 nion. There is probably some truth in each 

 of these explanations but we are not yet 

 able to estimate their relative importance 

 in the major problem. In each case the 

 utilization of oxygen was accomplished 

 through the agency of bacteria. Recent 

 studies have demonstrated large numbers 

 of bacteria in the lake water, e.g., Kus- 

 netzow and Karsinkin (1931). In view of 

 the large numbers of bacteria in the hypo- 

 limnion, there would seem to be greater 

 possibility that the original explanation 

 was the correct one, i.e., that hypolimnial 

 consumption of oxygen is largely due to 

 decomposition of dead plankton falling 

 through the hypolimnion. We need to 

 know\, however, whether these bacteria are 

 promoting oxidative decompositions and to 

 what extent they are acting on organic 

 matter or on the above mentioned gases. 



Even if the hypolimnial deficit should be 

 reasonably established as an index of pro- 

 duction there are several factors which 

 hinder or prevent its use in particular lakes. 

 If appreciable amounts of allochthonous 

 organic materials are present in the lake 

 water the utilization of oxygen in the de- 

 composition of these materials is a disturb- 

 ing factor of unknown extent. Thus in 

 cases where such dystrophic influences are 

 demonstrated or suspected w^e should not 

 attempt to apply the oxygen indices.^ 

 There are also some lakes in which there is 

 an appreciable production of oxygen in the 



1 Nevertheless Lonnerblad (1931) considers Thie- 

 iieniann 's calculations applicable to dystrophic lakes 

 which he studied, and Str0m suggests that the hypo- 

 limnial deficit may be used but should be expressed 

 per unit of volume rather than of area. Pre- 

 sumably these authors are not using the deficits as 

 indicating production. 



hypolimnion by photosynthesis. In Crys- 

 tal Lake, Wisconsin, Juday and Birge 

 (1932) found a high transparency in the 

 water and a green plant living on the bot- 

 tom at a depth of 20 meters, while in Crater 

 Lake, Oregon, Hasler (1938) records green 

 aquatic mosses from a record depth of 120 

 meters. Again, as Str0m (1931) points 

 out, the hypolimnial deficit ceases to indi- 

 cate production in extremely eutrophic 

 lakes where the hypolimnial oxygen is ex- 

 hausted and is of doubtful value in ex- 

 tremely oligotrophic lakes where the deficit 

 is very slight. Hutchinson (1938) has at- 

 tempted to meet the former objection by 

 the calculation of his real deficit as de- 

 scribed above. Finally, the consumption 

 of oxygen by respiration may at times have 

 a significant effect on the oxygen deficit, as 

 in the case of Lake Nipissing (Fry 1937) 

 where a large population of ciscoes is 

 crowded into a restricted hypolimnion. 



From the foregoing discussion it would 

 seem almost impossible to justify on theo- 

 retical grounds the use of the hypolimnial 

 deficit as a measure of production. Never- 

 theless, in the practical field we find it 

 used with apparent success by Hutchinson 

 (1938). He has demonstrated that, ap- 

 plied with care and discretion, it works 

 both in comparing productivity between 

 individual lakes and as an indication of the 

 character of lakes in a particular district. 



Having considered the question of hypo- 

 limnial oxygen deficits to this length, we 

 might conclude that, although they are not 

 easy to apply and subject to certain limita- 

 tions, they are of definite value qualitatively 

 and to a lesser extent quantitatively. 



In considering the controversial question 

 of oxygen as an index to lake metabolism 

 we must not neglect its direct effect on the 

 life of the lake through its availability for 

 respiratory requirements. 



In the epilimnion of a lake, because of 

 wind-caused renewal and photosynthetic 

 production, there is usually an abundant 

 supply of oxygen for respiratory needs. In 

 the hypolimnion we find a reduction in the 

 amount of oxygen varying from a slight 

 deficit to complete absence as described 



