above. Fish were acclimated to test temperature for 1.5 to 2 days prior to 

 introduction of ammonia toxicant. Ninety-six hour LC50 values ranged be- 

 tween 0.6-1.2 mg/liter NH3, but there was no correlation between ammonia 

 toxicity and temperature. Statistical treatment showed that size was not a 

 factor. We also conducted nine similar tests on 1-g cutthroat trout (S. 

 clarki ), within the range 13-19 C. Ninety-six hour LC50 values ranged be- 

 tween 1.0-1.5 mg/liter NH3, but again there was no temperature/ammonia toxi- 

 city relationship. In 15 tests on fathead minnows ( Pimephales promelas ), 

 however, within the range 13-22 C, we did find a definite correlation be- 

 tween temperature and susceptibility to ammonia toxicity. The toxicity 

 curves for these tests are shown in Figure 2. As temperature decreased, 

 toxicity increased. A plot of 96-hour LC50 values (mg/liter NH3) vs. tem- 

 perature, and a statistically computed correlation curve are illustrated in 

 Figure 3. It should be noted that in the case of the two trout species 

 tested, the temperature range studied was above their normal environmental 

 temperature; in the case of the fathead minnows, the range tested reached 

 several degrees below that for their optimum growth. We have not tested 

 fathead minnows at temperatures above, nor have we tested trout below, their 

 optimum growth temperature ranges. 



Our results for trouts agree with those reported by other researchers 

 within the temperature range 10-20 C (Herbert 1962; Lloyd and Orr 1969). 

 The British Ministry of Technology (1968), however, has reported that the 

 toxicity of ammonia to both adult and juvenile rainbow trout was much 

 greater at 5 C than at 18 C. Based on our analysis of their data as re- 

 ported, their case for juvenile trout appears stronger than that for adults. 

 The European Inland Fisheries Advisory Commission (1970) has cautioned that 

 acceptable concentrations of ammonia may be less at temperatures below 5 C. 

 Although this temperature value may be arbitrary, we conclude that there is 

 some merit to the argument that a drop in temperature below some optimum 

 range for a given species of fish may increase its susceptibility to ammonia 

 toxicity. It is important that this relationship be further studied. The 

 available evidence that temperature, independent of its role in the aqueous 

 ammonia equilibrium, affects the toxicity of ammonia to fishes argues for 

 further consideration of the temperature/ammonia toxicity relationship. 



EFFECT OF DISSOLVED OXYGEN 



The discharge of ammonia is frequently associated with a reduction in 

 oxygen levels in the receiving water. This is brought about by any of 

 several causes, including the oxygen demand of the ammonia itself as it is 

 converted by natural microbial oxidation to nitrite and nitrate; the chemi- 

 cal and biological oxygen demand of other chemicals which may be, and fre- 

 quently are, discharged along with ammonia; and ihe reduction in oxygen- 

 carrying capacity of the receiving water if the discharge causes a rise in 

 its temperature. If the receiving water body is rich in nutrients and 

 highly productive, as is frequently the case downstream from a sewage treat- 

 ment plant, there is the effect of diurnal and seasonal fluctuations in dis- 

 solved oxygen caused by plant growth. 



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