The Environment and Artificial Enrichment 



Ponds and lakes differ in size, depth^ and degree of control that can 

 be exerted over many variables. Increasing size and dapth imply decreas- 

 ing productive capacity (relatively) ^ and decreasing economic feasibility 

 for fertilization (Hasler, 19ii75 Smith., 1952). There are obvious limita- 

 tions in depth and area of a f ertilizable lake. The inability to control 

 several important factors is also discouragingo Added nutrients may be 

 carried away in lakes having a rapid rate of exchange. Affluents may bring 

 allochthonous nutrients into a lake in beneficial quantitieSj but may also 

 carry toxic substances and excessive nutrients to cause eutrophication. Most 

 lakes cannot be drained to derive the many benefits attributable to that 

 process o~/ Plant and animal populations are established, and controllable 

 only to a limited extent. This is disturbing in that the populations not 

 only affect fertilization, but may be affected by it in various and some- 

 times undesirable ways. Fluctuations in abundance of organisms irrespective 

 of enrichment are difficult to fathom, and insert a question into the inter- 

 pretation of fertilizer effects. Finally, the crop removal is rather indefi- 

 nite, being subject to the pressure and effectiveness of angling. 



Thermal stratification, for purposes of fertilization, divides a lake 

 into a trophogenic epilimnion and an oxygen-deficient, tropholytic hypo- 

 limnion which may entomb nutrient elements by permanent sedimentation. 

 Hutchinson and Bowen (19^7) found that U? percent of the phosphorus intro- 

 duced into a small lake had descended below the thermocline before it could 

 be utilized in the biological cycle. The fall overturn serves to disperse 

 oxygen and dissolved substances throughout the lake, and thus reverse the 

 tropholytic processes occurring in the hypolimnion during summer stagnation.. 

 However, it may also cause the precipitation and consequent loss of valuable 

 nutrients. The spring overturn acts similarly, but since it follows mild 

 winter activity, nutrient loss is not as great. The subsequent warming of 

 water in the presence of abundant nutrients causes increasing biological 

 activity. The thermocline, then, is significant because it diverts or 

 consumes many nutrients which would otherwise contribute to the productive 

 metabolism of the lake. 



It would seem desirable here to discuss lakes in the common (but 

 somewhat arbitraty) grouping of oligotrophia, eutroohic, and dystrophic. 

 By doing so, many related chemical, physical, and biological factors 

 could be considered in conjunction with results of lake= enrichment attempts. 

 However, supporting data in reports of lake fertilization are not detailed 

 enough to warrant this distinction,, Reports are summarized below in t.wo 

 groups s lakes supporting warm water fishes (warm-water lakes), and lakes 

 supporting salraonids (cold-water lakes) . Herein the separation is purely 

 mechanical J it may be of significance when more lake experiments have been 



2/ Certain impoundments are capable of being drained. In such circiunstances, 

 an increase in productivity has been noted after dry fallow^ refilling, 

 and restocking. 



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