Chu (19h3) concluded from laboratory results that nitrogen and phos- 

 phorus occur naturally in quantities far below the upper limit for optimal 

 growth and often do not reach lower optimal concentrations. These ele- 

 ments, he added, may limit growth at certain times of the year and may 

 exert a selective influence on different species of algae when concentra- 

 tions are below the lower optimal limit (nitrogen, 0.3 to 1.3 p. p.m. 

 phosphorus, 0.018 to O.090 p. p.m.). Such concentration limits may not 

 apply to natural situations. Since water is exposed to unlimited quanti- 

 ties of atmospheric nitrogen, Neess (19h9) postulated that nitro.-^en utili- 

 zation is limited only within the aforernentioned cycle.^ which is a system 

 in equilibriu;n, and does not depend upon niirogen addition. There is a 

 possible inverse relation of nitrate-nitrogen and direct relation of organic- 

 nitrogen content of water with productivity (Surber, 19^7). Nitrite nitro- 

 gen and ammonia are indicators of pollution because they posses an oxygen 

 demand (^iS-ebe, 1929). Aerobic conditions in the environment suppress a.m- 

 morda and improve conditions for nitrification. 



The value of potassium as a nutrient addendum is also questionable. 

 V/elch (1935) considered it a fixed requirement for plants in food manu- 

 cature and a catalyst occurring, naturally in small amounts (0.5 to 9.0 

 p.p.m, - Moyle, I9I49) . A slight acid reaction results when its compounds 

 are added to water (Wiesner, 1937) • Swingle and Smith (1939a) found that 

 small amounts of potassium increased pond yields^ but lari,er quantities 

 caused no further increases. Potassium has a most favorable influence in 

 peat,, sand, and hard-bottomed ponds j in mud ponds, it inhibits hard water- 

 flora (such as Equisetum ) and favors soft waterflora (SchaeperclauS;, 1933) . 

 Neess (I9li9) stated that results of potassium fertilization are erratic 

 but cited an instance of increased 'production tiy use of potassium alone He 

 concluded that the effects of potassium are indirect, selective, and partly 

 bacteriological . 



Compounds of calcium and magnesium, for the most part, function simi- 

 larly in water metabolism. As an individual element, calcium is the more 

 abundant and important of the two, often occurring naturally in large quan- 

 tities. Welch (1935) considered calcium in the following roles i (1) 

 Related to the translocation of carbohydrates 3 (2) an integral component 

 of plant tissue] (3) acts to increase the availability of other ionsj (U) 

 reduces toxic effects of single-salt solutions of other elements. Its 

 presence is obvious in some animal tissue, especially the exc skeletons of 

 arthropods and mollusks. Magnesium, Welch stated, is a component of 

 chlorophyll and, in some instances, acts as a carrier of phosphorus, Wunder 

 et al. (1936) stated that magensium stimulater^ bacterial reduction of 

 organic matter in the bottora„ Cal':ium- rich waters are those draining marl 

 and limestone soils. Schaeperclaus (1933) claimed that he had never 

 encountered a pond too rich in calcium. Surber (19U5) believed waters that 

 acquire hardness by contact with limestone formations may foster growth of 

 Chara that reach great density and curtail fish production. Schaeperclaus 

 (193J) and Welch (1935) discussed the relations of calcium and magnesium 

 to the carbon-dioxide mechanism of water in some detail. The affinity of 



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