I 



these elements for free carbon dioxide results in the formation of soluble 

 bicarbonates (CA/JcoJL) or half-bound carbon dioxide, and carbonates 

 (CaCO-s) or fixed carbon dioxide. This mechanism is somewhat complex, being 

 related to acidity-j buffering, photosynthetic action, removal of decomposi- 

 tion products, and activation of other nutrients. Calcium may be deposited 

 in large quantities as lime on the bottom of lakes and ponds » Mollusks and 

 certain algae (marl-forming organisms) are important organisms functioning 

 in this process. Calcium nutrition will be considered later in the dis- 

 cussion of liming. 



Conflicting opinion is found in the literature on the value of iron in 

 fresh waters. High iron content, according to Schaeperclaus (1933) <• is a 

 phenomenon usually accompanying acid waters, and it presence denotes a 

 poorly productive habitat, Wunder et al. (1936) believed ponds rich in 

 iron and aluminum were good producers and that typically good ponds had 

 bottom soils with autochthonous organic sediments rich in iron and manga- 

 nese. General belief holds that a small amount of iron is necessary^ but 

 that large quantities are detrimental to productivity. Welch (1935) indi- 

 cated that the function of iron lies in chlorophyll production, and that 

 it possibly acts as a catalyst or oxygen carrier. The best algal growth, 

 he stated, is in water having 0.2 to 2,0 milligrams of ferric oxide per 

 liter, but in the absence of buffer compounds 5 milligrams per liter may 

 be toxic. Estimation of the iron content of water may be made from the 

 amount of ferric mud in the bottom (Schaeperclaus, 1933)- The power of 

 iron as a reducing agent accounts for its undesirable activity. 



Welch (1935) briefly described the importance of several minor elements. 

 Silicon is a structural component of diatoms and sponges, being soluble in 

 water as silicate. Blooms of diatoms or desmids may cause high fluctuations 

 in the content of dissolved silicon. Sodium supplements the action of 

 potassium and may act as an antidote against the toxicity of some salts. 

 Sulfur is necessary to protoplasm as a constituent of certain amino acids j 

 it may occur abundantly in organic matter or combined with iron in bottom 

 soils. Manganese is esrential in minute quantities to chlorophyll -bearing 

 plants. Zinc and copper are required by protoplasm in small amounts. The 

 former may stimralate plant growth, but both may be toxic in large quanti- 

 ties. This group of elements, with the exception of sodium and sulfur, 

 occur sparingly in natural waters, and little is known about their optimal 

 levels or roles in water metabolism. 



Organic compounds are more varied and complex than mineral substances. 

 As previously mentioned, some of the organic matter in lakes and ponds occurs 

 in solution. Welch (1935) discussed the composition and function of this 

 dissolved matter. It is a mixture of many substances (carbohydrates, fats, 

 and proteins), the most prominent being nitrogenous waste products. In 

 natural waters the total amount of dissolved organic matter usually ex- 

 ceeds 10 milligrams per liter. Nutritionally, it is known that some 

 organisms exist mainly on such dissolved material. Largest quantities of 

 undissolved organic matter are found as humus in t he substrate, reaching 



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