independent of fertilization. The objective of artificial enrich- 

 ment is productive increment, considered here in tenns of yield of 

 desirable fish per unit area and time, 



5c Chemical considerations of the environment concern the presence 

 and reactions of nutrient elements or compounds, and their rela- 

 tions to living organisms. Since the substrate is chemically 

 and biologically active in shallow water, fertile soils signify 

 high productivityo Productive waters are generally rich in dis- 

 solved substances and have an alkaline pH (7.0 to 8.5). Many 

 chemical elements are required for the sustenance of life but only 

 four (nitrogenc, phosphorus, potassium, calcium) and organic matter 

 have been widely used in aquatic fertilization. Whether these ele- 

 ments limit productivity is a controversial matter, but most 

 authorities agree that phosphorus is usually limiting in natural 

 waters, and is the most valuable fertilizing element. 



6. Phosphorus functions as an assimilator of nitrogen into cellular 

 material o It occurs naturally in a dynamic state, the dissolved 

 component (organic and phosphate phosphorus) being smaller in 

 quantity with a consequent faster turnover rate than the phosphorus 

 in solids. Soluble phosphate is absorbed directly in a matter of 

 minutes by lower organisms, and can be concentrated in large 

 amounts by fauna. Since it is an active element, phosphorus may 

 confcine with iron or calcium and be lost by permanent sedimentation 

 - of insoluble phosphates. 



I. Nitrogen, a constituent of protein, is found free and combined 



(NH3, NO25 NO3, and organic nitrogen) in the water. These forms of 

 nitrogen are related in a cycle energized by bacterial activity. 

 Nitrate is generally regarded as available nitrogen, but lower 

 organisms may also utilize nitrite and ammonia nitrogen. 



80 Calcium and magnesium function similarly in the, complex carbon- 

 dioxide mechanism of thewater. Individually, calcium i s the more 

 in^jortant element, often the major precipitate and dissolved 

 cation in waters draining lime-rich soils. It functions physio- 

 logically in plant tissue, is a prominent structural member in 

 faunal groups, and, in general, ameliorates chemical conditions 

 in the environment. Magnesium is not so abundant, but is neces- 

 sary for chlorophyll production and may aid bacterial reduction 

 in the substrate. 



9. Potassium is the most beneficial of the remaining elements, 



especially to submerged flora. As a fertilizer, its effects are 

 indirect and selective, and its reactions are most favorable in 

 bottoms of peat or sand. Functions of other mineral elements are 

 discussed. 



28 



