PHYSICAL AND CHEMICAL FACTORS IN THE METABOLISM OF LAKES 



17 



above. In the thermocline the (inantities 

 of oxygen sliow considerable differences. 

 The thermoclinal niinimnm or oxygen notcli 

 lias been observed freqnently and at the 

 other extreme Juday and Birge (1932) have 

 demonstrated an excess of oxygen in some 

 cases anioinitiiig to extreme snpersatura- 

 tion. The excess is accounted for by active 

 photosynthesis of the phytoplankton, bnt 

 the complete explanation of the tliermo- 

 clinal minimnm is not yet available. Most 

 explanations assume that the dead and dy- 

 ing plankton sinking from the epilimnion 

 tend to linger and accumulate in this zone 

 and that the notch indicates tlie decomposi- 

 tion of this material. The use of oxygen 

 for respiration by zooplankton is undoubt- 

 edly involved but it is considered inade- 

 (piate (Naber 1933) to account for the mini- 

 mum. To explain the delay in the sinking 

 of organic material. Birge and Juday 

 (1911) have cited the sharp decrease in 

 temperature and resulting increase in den- 

 sity at this level. Antonescu (1931) dis- 

 cusses the effect of currents on sinking 

 bodies and thinks the delay related to the 

 ''minimum turbulence" in the thermocline. 



Considering tlie distribution of dissolved 

 oxygen in the three regions it may be as- 

 sumed tliat it is rarely a limiting factor for 

 distribution or j^roduction in the epilim- 

 nion, tliat it may operate in an irregular 

 way in the thermocline, and that in the 

 hypolimnion the deficiency of oxygen fre- 

 quently tends to drive out certain animal 

 forms. 



Field observation of the effects of lowered 

 oxygen on animal distribution has been ex- 

 tensive, but unfortunately it is often im- 

 possible to separate the effects of oxygen 

 shortage from those of other factors, for 

 example, increased carbon dioxide or hy- 

 drogen sulphide. In some cases the distri- 

 bution of fish seems to give clear-cut evi- 

 dence of degrees of oxygen tolerance, for 

 example, the distribution of the lake trout 

 in lakes of Prince Albert National Park, 

 Saskatchewan (Rawson 1936). In the 

 pioneer experiments of Smith (1925) in 

 Douglas Lake, the experimental fish failed 

 to live below the thermocline where the 



oxygen was severely reduced. As he points 

 out we do not know whether lack of oxygen 

 was the only or just the chief factor. Scott 

 (1931) describes the cisco in Indiana lakes 

 as forced up by oxygen depletion and 

 trapped beneath the still lower concentra- 

 tions of a thermoclinal minimum. Fry 

 (1937) attributed the upward migration of 

 ciscoes in Lake Nipissing to the combined 

 ett'ect of lowered oxygen and increased car- 

 bon dioxide. However, in view of the effect 

 of high carbon dioxide on the buoyancy 

 mechanism of certain fishes (Hall 1924, Fry 

 ft (il. 1937), it is possible that the upward 

 migration is more closely related to the 

 increase in carbon dioxide tension which is 

 concomitant with the lowering of oxygen in 

 the hypolimnion. 



In cases of extreme stagnation the effects 

 of hydrogen sulphide may be added to those 

 of high carbon dioxide and low oxygen. 

 In Pinantan Lake, observed by the author 

 (Rawson 1934), there was no oxygen below 

 the thermocline. The fish (trout) had 

 moved out and even the oligochaetes, 

 chironomid larvae, and pisidia, normally re- 

 sistant to lowered oxygen, were absent from 

 the bottom of the hypolimnion. It is prob- 

 able that the hydrogen sulphide present was 

 in some measure responsible for the for- 

 mation of what AVelch (1935) terms a "bio- 

 logical desert" below the thermocline. 



In suggesting that further work is needed 

 in the field we should mention also certain 

 experimental work which has shown inter- 

 esting possibilities. Powers (1934) has in- 

 dicated the possible nature of the effect of 

 free carbon dioxide on the ability of haemo- 

 globin to acquire oxygen. Laugford (1938) 

 drew certain conclusions as to the effect of 

 dissolved oxygen, carbon dioxide, and tem- 

 perature on migration of entomostraca in 

 Lake Nipissing. He then tested these con- 

 clusions in the laboratory using apparatus 

 devised by Fry to reproduce in miniature 

 the gradients observed in nature and was 

 able to some extent to verify his first the- 

 ories. A comparable device for laboratory 

 studies has been described recently by 

 Schwabe (1937). 



Tlie differences in the oxygen require- 



