20 



PROBLEMS OF LAKE BIOLOGY 



observation that if there are no macroscopic 

 lime encrustations visible (on plants, etc.) 

 the Avater is poor in calcium, if visible 

 medium, and if abundant encrustations and 

 concretions the lake is rich in calcium. 



The significance of calcium content with 

 respect to trophic condition and the classi- 

 fication of lakes has been much discussed. 

 The typical eutrophic lake is rich in cal- 

 cium and the typical oliyotrophic lake 

 usually poor. However, Str0m (1928) 

 pointed out that certain oligotrophic lakes 

 in the Alps were rich in calcium and he pro- 

 posed a subdivision of each of the primary 

 types into calcium rich and calcium poor 

 divisions (Fig. 5). Ohle (1934) has since 



Fig. 5. Calcium and humus in the division of x'li- 

 mary lake types. (From Str0m, 1928.) 



reported lakes in northern Germany eutro- 

 phic but poor in calcium. Naumann 

 (1932) terms certain oligotrophic lakes of 

 the Alps rich in calcium "alkalitrophic," 

 an opinion with which Thienemann (1933) 

 strongly disagrees. 



NITROGEN AND PHOSPHORUS 



Nitrogen and phosphorus are raw mate- 

 rials for protein synthesis and thus stand 

 in a position as important as that of carbon 

 dioxide in photosynthesis. It would seem 

 that a study of the availability of nitrogen 

 and phosphorus should be a direct ap- 

 proach to the evasive problem of productiv- 

 ity but the approach has many obstacles. 

 It is difficult to obtain satisfactory deter- 

 minations of the amounts available ; our 

 knowledge of the role of bacteria in the 

 cycle is incomplete and the greatest diffi- 

 culty is our inability to measure the rates 

 of transfer and utilization. 



The amounts of inorganic nitrogen in 

 natural waters are relatively small with 



nitrates usually less than 0.5 mg per liter, 

 nitrites less than 0.1 and ammonia also 

 scarce but capable of considerable increase 

 if oxygen is reduced. Changes in seasonal 

 and vertical distribution are so marked that 

 extensive sampling is required. After the 

 usual maximum in late winter, vernal cir- 

 culation brings about a fairly uniform dis- 

 tribution and then the phytoplankton in 

 the upper layers begins to reduce the 

 supply. Dead organisms sinking and de- 

 composing increase the amounts near bot- 

 tom until at midsummer nitrates near bot- 

 tom are often several times that at the 

 surface. Nitrites and ammonia may also be 

 concentrated at the bottom and are fre- 

 quently absent at the surface. Autumn cir- 

 culation again restores uniform distribu- 

 tion and the decomposition of the summer 

 "crop" of organisms begins to increase the 

 inorganic nitrogen again toward the winter 

 maximum. The extent of stratification and 

 variation of inorganic nitrogen has been 

 demonstrated by Yoshimura (1932), by 

 Juday et al. (1938) , and by others. A fuller 

 explanation of these changes in terms of 

 water movements, the activities of green 

 plants and bacteria lies beyond our scope. 



The average amounts of phosphorus in 

 natural waters is usually less than .05 mg 

 per liter and thus much less than those of 

 nitrogen. During the summer there is 

 often a marked increase in the phosphorus 

 of bottom ^^ater and sometimes but not 

 always a decrease at the surface (Juday 

 and Birge 1931). The cycle involving con- 

 centration near bottom and subsequent dis- 

 tribution by water circulation is similar to 

 that of nitrogen. Certain European inves- 

 tigators have given their attention recently 

 to the loss of phosphorus from circulation 

 either temporarily by adsorption to col- 

 loidal humous material (Ohle 1935, Gess- 

 ner 1934), or permanently by precipitation 

 as iron phosphate (Einsele 1936, 1938). 

 The adsorption of phosphate to iron hy- 

 droxide is suggested by Gessner (1934a) 

 as causing a physiological scarcity of phos- 

 phates. 



The small quantities of nitrogen and 

 phosphorus in lake waters, their variability 

 and the technical difficulties of their deter- 



