326 MARSHALL AND MELLINGER 



cladoceran and rotifer populations were more sensitive to copper 

 than the cyclopoid copepods. In a laboratory study of the acute 

 toxicity of cadmium and other heavy metals to three species of 

 crustacean zooplankton, Baudouin and Scoppa (1974) found that 

 the cladoceran Daphnia hyalina was considerably more sensitive than 

 the two copepod species and that the calanoid Eudiaptomus padanus 

 was more sensitive than the cyclopoid Cyclops abyssorum. They 

 found only minor differences in the rank orders of the metals' 

 toxicities to these species, and their results for D. hyalina were in 

 good agreement with those reported for D. magna by Biesinger and 

 Christensen (1972). The observed similarity of cadmium effects on 

 different species of Cladocera agrees with the findings of Winner and 

 Farrell (1976) that four species of Daphnia did not differ in their 

 sensitivity to copper. 



The observed effects of cadmium on structural attributes of a 

 Lake Michigan zooplankton community are qualitatively similar to 

 those found in studies of long-term effects of other stresses in natural 

 communities (Woodwell, 1970). The long-term effects of cadmium 

 on the Lake Michigan zooplankton community would probably be 

 more pronounced than the effects observed in this study, however, 

 because heavy-metal toxicity tends to increase with chronic expo- 

 sure. For example, chronic cadmium toxicity to laboratory popula- 

 tions of Daphnia galeata mendotae increases with time, and 23 weeks 

 are required to establish equilibrium relationships (Marshall, 1978). 



The various indexes of community structure differed consider- 

 ably in their sensitivity to cadmium. Total zooplankton abundance, 

 the most sensitive index, was reduced by 42% in 9 days at 5 jug 

 Cd /liter, whereas the most sensitive indexes of species diversity and 

 community similarity, N2 and S', were reduced by only 29 and 

 27%, respectively. The diversity number N2 was the most sensitive 

 index of species diversity because it is the one most heavily weighted 

 by the more abundant species (Hill, 1973). This also explains the 

 different sensitivities of the two indexes of similarity, S and S'. The 

 indexes of similarity account for changes in relative proportions of 

 each species, whereas the diversity number accounts only for 

 presence or absence of species. Although total abundance was the 

 most sensitive index of community structure in our study, species 

 diversity or community similarity would probably be more sensitive 

 to long-term effects of chronic, low-level cadmium stress because 

 total zooplankton abundance would probably be partially restored, 

 given time, by more tolerant species. 



There are few previous studies of heavy-metal effects on aquatic 

 ecosystems with which to compare our results, and none dealing with 



