AN IN SITU STUDY OF CADMIUM STRESS 

 IN A NATURAL ZOOPLANKTON COMMUNITY 



J. S. MARSHALL and D. L. MELLINGER 



Ecological Sciences Section, Radiological and Environmental Research 



Division, Argonne National Laboratory, Argonne, Illinois 



ABSTRACT 



The effects of elevated cadmium concentrations on a natural zooplankton 

 community were studied in situ in Green Bay, Lake Michigan, during the 

 summer of 1976, with polyethylene carboys as enclosures. The results of five 

 experiments showed that cadmium concentrations as low as 5 /Jg/liter, the 

 lowest concentration tested, caused large and highly significant effects 

 (P < 0.001) on both functional and structural attributes of the zooplankton 

 community within 9 days. The effects of enclosure up to 15 days were mostly 

 insignificant. Functional responses, as indicated by changes in the different 

 species' rates of increase, showed highly significant differences (P < 0.001) 

 among total Cladocera, Calanoida, and Cyclopoida, but no differences among 

 species in each of these major groups. Structural responses included reduced 

 total abundance, species diversity (3 indexes), and community similarity 

 (2 indexes). The results indicate that added cadmium concentrations much lower 

 than 5 /Jg/liter probably would cause detectable effects within 9 days and that 

 incubations up to 15 days are feasible. 



The need for experimental studies of pollutant stress in whole 

 communities and ecosystems, as opposed to lower levels of organiza- 

 tion, has been emphasized recently by several ecologists (Barrett, 

 Van Dyne, and Odum, 1976; Jernelov and Rosenberg, 1976; Fisher 

 and Wurster, 1974). The usefulness of in situ experiments in studies 

 of toxic stress in a marine ecosystem was shown by Reeve et al. 

 (1976), who concluded that the zooplankton community was 

 probably the component most affected. Although it has been 

 possible to maintain nearly normal zooplankton communities in 



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