Stroganov (1975) recommends as acceptable the use of toxins that produce 

 not more than 25 percent mortality. The equations present here calculate 

 the data of death of 25 percent of Daphnia (T25). As a rule, interpolations 

 have been made, but extrapolation is also possible. 



Practically, it is important to determine the minimum time period of ob- 

 servation that is sufficient for reliable calculations. To this end, re- 

 gression equations for various time/mortality points were calculated. This 

 enables a determination of the number of time points that would be suffi- 

 cient for calculation of the T25, value that does not differ significantly 

 from the experimental value for 30 days. In Table 2 the dependence of cor- 

 relation coefficients and the T25 value from the length of experiment is 

 shown. The value of T25 does not significantly change for different periods 

 of observation. This information makes it possible to limit the duration of 

 experiments. For the calculation of coefficients for the equation, two dots 

 are enough, but the reliability of calculated values will be low. For reli- 

 able results, it is advisable to have 3 to 4 time/mortality points for every 

 concentration. In relatively high concentrations and with frequent record- 

 ing of results, the time period can be \jery short. Thus, experimental re- 

 sults can be completed and quickly specify a preliminary assessment of 

 acceptable concentrations. 



As a result of these calculations a set of data is available that 

 characterize the time of death of test organisms in varying concentrations 

 (Table 3). The graphical relationship of concentration to time of death of 

 25 percent of Daphnia can be given as shown in the Figure 4A. This rela- 

 tionship can also be described by regression equations. From examined re- 

 gularities (exponential, power, logarithmic and hyperbolic) the power func- 

 tion was found to be most suitable (Table 4). The correlation coefficients 

 for the power function are highest, and it can be simply calculated by usual 

 methods. This function is also suitable from a logical standpoint. Indeed, 

 the curve of this function can never cross the axes, because time cannot be 

 negative function, and there are enough small concentrations that do not in- 

 fluence the life-span of Daphnia . The concentration that does not effect 

 Daphnia corresponds to the vertical asimptote. 



There is certain diversity in the relationship of concentrations of pol- 

 lutants to their effects (Warren 1971). However, these relationships can be 

 described with a high degree of approximation by power or other simple func- 

 tions. Using logarithmic axes, the power function becomes a straight line 

 (Figure 48), and approximate equation coefficients can be calculated from 

 two concentrations. 



By using these equations for certain compounds, we can evaluate the time 

 of death for other concentrations, and estimate the concentration that 

 causes the death of 25 percent of Daphnia in a given time period. The 

 period of life-span can be limited to 30 days, and mortality to 25 percent. 

 The concentration, that corresponds to these data, will be an acceptable 

 concentration in terms of survival (Table 5). In this table the acceptable 

 concentrations were calculated from data of concentrations, and a comparison 

 with values that were accepted from experimental evidence is made. It is 

 natural that there are some differences between experimental data and 



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