AN IN SITU STUDY OF CADMIUM STRESS 317 



enormous enclosures (18,000 m-^) up to 22 months in a small lake 

 (Smyly, 1976), the use of large enclosures in the Great Lakes has 

 been limited by their high unit cost and susceptibility to storm 

 damage (Schelske and Stoermer, 1972). 



Cadmium has long been recognized as a highly toxic element, but 

 only recently has concern been expressed over the impact of 

 anthropogenic emissions of cadmium into the environment. Man's 

 activities now appear to be contributing more cadmium to streams 

 than is contributed by natural processes (Fleischer et al., 1974). 

 Unfortunately, the concentrations and distribution of cadmium in 

 the Great Lakes remain poorly knovm. The average concentration of 

 cadmium in Lake Ontario is 0.09 /jg/liter (Chau et al., 1970). In the 

 heavily polluted, extreme southern portion of Green Bay, Lake 

 Michigan, cadmium concentrations ranged from 1.0 to 1.4 ^ig/liter 

 during 1973 (Wiersma et al., 1974). Recent analyses at Argonne 

 National Laboratory indicate, however, that the concentration of 

 cadmium in the main body of Lake Michigan is less than 0.1 )Ug/liter 

 (Tisue, 1976). 



The purpose of our study was to determine the short-term effects 

 of increased cadmium concentrations on the zooplankton compo- 

 nent of the Lake Michigan ecosystem. We used an in situ method 

 designed to overcome some of the limitations of large enclosures for 

 experiments in large lakes. Following guidelines recommended by 

 Barrett, Veqi Dyne, and Odum (1976) for ecosystem stress studies, 

 we studied both functional and structural attributes of the zooplank- 

 ton component under seminatural conditions and evaluated the 

 relative sensitivities of different indexes of stress. 



MATERIALS AND METHODS 



Field Methods 



During the summer of 1976, five in situ cadmium-enrichment 

 experiments were conducted in northern Green Bay, Lake Michigan. 

 Two sampling and incubation stations were located where the 

 bottom depth is approximately 30 m, 1 km northwest of the larger 

 of the Sister Islands off Sister Bay, Wisconsin. Water was collected 

 from different depths in the lower half of the epilimnion (6 to 12 m) 

 with a nonmetaillic, 16-liter Kemmerer bottle and composited in a 

 large mixing tub. In each experiment, two to four opaque polyethyl- 

 ene carboys (8- or 20-liter capacity) for each of five levels of added 

 CdCl2 were filled gradually with water from the mixing tub by 



