AN IN SITU STUDY OF CADMIUM STRESS 319 



Three indexes of species diversity were calculated according to 

 HUl (1973): 



Nq = total number of identification categories (2) 



Ni =exp(-i: Pi In Pi) (3) 



'^? (4) 



N 



1 



where Pi is the proportional abundance of the different identification 

 categories in a given sample. 



Two indexes of community similarity, S and S , were calculated 

 according to Whittaker (1972): 



2C 



A + B 



(5) 



where A is the number of identification categories in the control 

 samples, B is the number of categories in any single sample, and C is 

 the number common to both; and 



S' = l-0.5 2:|APi| (6) 



where APi is the difference between the proportional abundance of a 

 given identification category in any single sample and its average 

 proportional abundance in the controls. 



RESULTS 



The effects of cadmium on functional attributes of the com- 

 munity, as indicated by the relationships among average rates of 

 increase (r) and added cadmium (micrograms per liter) for the 

 predominant zooplankton groups and component species, were more 

 consistent and much more pronounced than the effects of enclosure, 

 which are discussed later. The linear regression coefficients (slopes) 

 for relationships among average rates of increase and added cadmium 

 (0 to 50 /ig/liter) for the three major groups of crustacean zoo- 

 plankton in each of the five experiments are shown in Table 1. 

 Friedman's analysis of variance by ranks (Siegel, 1956) indicates that 

 the differences between slopes for cladocerans, calanoid copepods, 

 and cyclopoid copepods are highly significant (P < 0.001). The mean 

 regression coefficients for the Cladocera, Calanoida, and Cyclopoida 



