(dissolved oxygen — 80^ saturation). In experiment 9a (Table 9) 



M. cephalus died somewhat faster in the non-aerated aliquot of a 



G. brevis culture thah in the aerated aliquot. It is apparent 



from these contradictory results that the influence of such factors 



as aeration cannot be evaluated until the standardization of the toxicity 



tests is more complete. 



Several factors probably influence the degree of toxicity of 

 G. brevis cultures. Aside from the concentration of Go brevis 

 other factors such as the growth phase of culture, pH of culture 

 during growth and during test period, temperature and salinity of 

 test culture, size and number of test fish, volume and degree of 

 aeration of test culture, and bacterial growth, are among those 

 which must be considered in standarizing the toxicity tests. 

 Shilo and Aschner (1953) found that a number of factors influenced 

 the toxicity of Prymnesium parvum cultures. The toxicity was 

 decreased by oxidizing agents, aeration, adsorbents including 

 pond-bottom' soils, bacteria growth, and low pH (below 7.5). 

 They improved the standardization of their toxicity tests by using 

 a buffer to control pH of the test culture and streptomycin to suppress 

 bacterial growth. Furthermore, the test cultures were not aerated o 

 McLaughlin (1956) working with the same organism reported that cultures 

 grown in an alkaline medium were more toxic thah those grown in an 

 acid medium, P. parvum cultures (grown in alkaline media) rendered 

 non- toxic by lowering the pH to 6.0 regain their toxicity when made 

 alkaline (Shilo and Aschner, 1953; McLaughlin, 1956), 



Some of the factors mentioned above may account for the variable 

 "death times" obtained in duplicate containers of G. brevis cultures. 

 Since on some occasions the pattern of response in one of the duplicate 

 containers was different from that in the other, we consider that 

 factors other than variations of the individual test fish were 

 responsible. For example, in experiment 6 (Table 4) the fish 

 died in 9 and 16 minutes ip one container whereas death occurred 

 after 1-2/3 and 2 hours exposure in another. Both containers 

 (non-aerated) held similar amounts of the same unialgal culture. 

 There are other anomalies which defy explanation at present. 



One of these anomalies concerns the variation in the response of 

 fish subjected to cultures supposedly grown under the same conditions, 

 and treated in the same manner throughout the study. For example, in 

 experiment 8 (Tables 6 and 7) container 9 received one of the duplicate 

 pure cultures and container 10 received the other,. The G, brevis 

 counts of the material in each of the containers and such measured 

 factors as pH, dissolved oxygen, and salinity were comparable. 

 The similarity between the two containers is further emphasized by 

 the "distress times" for the M„ cephalus — 10 and 12 minutes for con- 

 tainer 9, 13 minutes for each of the two fish in container 10, In 

 spite of all these similarities, the M. cephalus in container 9 died 



45 



