22 



PROBLEMS OF LAKE BIOLOGY 



of cliironomids so commonly used as indica- 

 tors of tropliic condition. Tlie authors 

 point out the uncertainty as to the relative 

 significance of redox potential and oxygen 

 content in affecting the distribution of these 

 larvae, but one might expect the potential 

 to provide a useful index to the hypolimnial 

 and benthic environments. 



Another special Avay in which iron enters 

 into the chemical economy of the lake has 

 been mentioned above where the presence 

 of iron hydroxide in the deeper water re- 

 sults in adsorption and precipitation of 

 phosphates thus removing them from cir- 

 culation. 



SILICA 



Silica in the surface waters of lakes is 

 iiighly variable but usually less than 10 mg 

 per liter. Stratification is frequent and the 

 bottom water may contain twice that of the 

 surface. 



The variation in silica content has been 

 compared with the amount of diatoms pro- 

 duced both in marine and fresh waters. 

 Atkins (1926) finds no proof that silica is 

 ever a factor limiting diatom production in 

 the ocean. Meloehe cf al. (1938) observed 

 certain correlations between silica and dia- 

 toms in Lake Meudota but concluded that 

 several factors were involved. Since the 

 silica content did not fall below .13 mg per 

 liter it was presumablj^ not limiting. 

 Kicker (1937) found 6.0 mg of silica per 

 liter in water of Cultus Lake and calculated 

 that this was about 25 times the amount of 

 silica represented by diatoms at the time of 

 their maximum abundance. 



ORGANIC IMATERIALS 



In the foregoing discussion the treatment 

 ■of chemical materials has been arbitrarily 

 limited to their mineralized state. Some of 

 these materials are being built up into liv- 

 ing matter and returned to the mineral 

 :state. Thus in the economy of the lake we 

 must consider also the materials repre- 

 :sented by living organisms and by the dead 

 organic matter at various stages of destruc- 

 tion. 



The origin of the non-living organic mat- 

 ter is twofold. From the point of view of 



productivity we have special interest in 

 those produced within the lake (autochtho- 

 nous) by the death of organisms or as physi- 

 ological wastes. Organic materials of exter- 

 nal (allochthonous) origin are introduced 

 from the surrounding terrain chiefly by 

 drainage or seepage. Unfortunately in 

 practice the two can not be so readily 

 separated. 



Birge and Juday (1934) have indicated 

 that the dissolved (non-centrifugable) or- 

 ganic material in Wisconsin lakes to be 

 composed of about 75 per cent carbohy- 

 drates, 25 per cent proteins, and a very 

 small quantity of fats. The carbon to ni- 

 trogen ratio was approximately 12 to 1. 

 The same authors found that in lakes with 

 a minimum of allochthonous materials the 

 total organic (dissolved and particulate) 

 material was about 4 mg per liter, of which 

 the plankton represented approximately 16 

 per cent. In the average of their lakes they 

 found 16 mg of which plankton was 8 per 

 cent, and in lakes rich in extractives 50 mg 

 and plankton 4 per cent. 



Assuming the existence in most lakes of 

 an amount of dissolved organic material 

 possibly seven times as great as that of the 

 plankton, the question arises as to how and 

 to what extent this is made available for 

 the production of living organisms. Pre- 

 sumably the chief factor is the action of 

 bacteria in breaking it down into nitrates 

 and ammonia. The study of lake bacteria 

 is advancing rapidly and we can soon hope 

 for more light on this question. A second 

 activity, the direct utilization of dissolved 

 organic materials by lower plants and ani- 

 mals is still debatable. It seems probable 

 from the work of Krogli (1931) that such 

 utilization occurs but is of little importance. 

 Nevertheless, as Varga (1934) indicates, 

 we cannot exclude it as a factor. 



That the dissolved organic materials are 

 by no means equally available in different 

 lakes is apparent from their abundance in 

 lakes strongly affected by dystrophy and 

 producing relatively smaller amounts of 

 plankton. In such lakes (with color 130- 

 268 U. S. Geol. Sur. scale) Birge and Juday 

 (1934) found the plankton as low as 3 per 

 cent of the total organic material while in 



