AN IN SITU STUDY OF CADMIUM STRESS 327 



the effects of cadmium. The most relevant study of heavy metal 

 stress is that of Reeve et al. (1976) on the short-term effects of 

 copper on a marine zooplankton community in enclosed v/ater 

 columns. Reduced abundance of total zooplankton was apparently 

 the most sensitive index of copper stress, just as it was for cadmium 

 in our study. Reeve and co-workers were not able to detect a 

 statistically significant reduction of either Shannon's index (H) or 

 percentage similarity (S') caused by copper concentrations up to 

 50 )Ug/liter because of pronounced enclosure effects. Total phyto- 

 plankton, as measured by chlorophyll a, increased in the copper- 

 treated enclosures because of reduced zooplankton grazing (Reeve et 

 al., 1976). The effects of various heavy metals and other pollutants 

 on functional and structural changes in freshwater algae and 

 protozoan communities include reduced total abundance and in- 

 creased relative abundance of tolerant species (Cairns, Lanza, and 

 Parker, 1972). The effects of copper have probably been studied 

 more than those of any other heavy metal because it is frequently 

 used as an aquatic herbicide, but there are few studies even of copper 

 effects on zooplankton (Mcintosh and Kevern, 1974). Heavy metals 

 associated with acid precipitation have been implicated in the 

 reduction of biomass and species diversity of phytoplankton, 

 zooplankton, and fish communities in lakes chronically exposed to 

 these inputs (Gorham, 1976). The well-documented reductions of 

 abundance and species diversity of zooplankton and fish communi- 

 ties in the LaCloche Mountain lakes near the mining and smelting 

 region of Sudbury, Ontario, appear to be caused primarily by acid 

 stress rather than by the associated heavy metals (Beamish, 1976; 

 Sprules, 1975). 



Reduced biomass and diversity appear to be common symptoms 

 of stress caused by many kinds of disturbances, at least in terrestrial 

 ecosystems (Woodwell, 1970). Not all stresses have the same effects, 

 however. Eutrophication is frequently referred to as a stress, yet its 

 effects are just the opposite of those of oligotrophication (Hutchin- 

 son, 1973), a process that is claimed to occur in lakes subjected to 

 excessive acid loading (Gralin, Hultberg, and Lander, 1974). Thus 

 different stress syndromes are more likely to be found for different 

 classes of stressors, e.g., toxic substances. Universally toxic sub- 

 stances, such as cadmium or strong acids, are even more likely to 

 have similar and predictable effects than are more-selective toxicants, 

 such as pesticides (Hurlbert, 1975). 



The effects of heavy metals and other toxic substances on 

 aquatic communities in the Great Lakes have been rated low in 

 importance relative to other stresses, including uncontrolled commer- 



