328 MARSHALL AND MELLINGER 



cial fishing, erosion and nutrient loading, invading species, and 

 stream destruction and shoreline restructuring (Regier and Hartman, 

 1973). Certainly any reduction of plankton abundance which may 

 have been caused by toxic substances in recent decades has been 

 masked by dramatic effects of eutrophication and fish predation. In 

 Lake Michigan, alewife predation has had a dominant influence on 

 changes in the zooplankton community (Wells, 1970) although 

 eutrophication may have been responsible for counteracting the 

 effects of alewife predation in Green Bay (Gannon, 1972). 

 Eutrophication has not only increased the total abundance of Lake 

 Michigan phytoplankton (Beeton, 1965) but has also changed the 

 species composition (Schelske and Stoermer, 1972). As long as such 

 dramatic changes are taking place in the Great Lakes, there seems to 

 be little chance of detecting the effects of toxic substances by direct 

 observation (monitoring), except in the vicinity of strong local 

 sources. 



The results of this study indicate that the use of relatively small 

 enclosures overcomes some of the limitations of large enclosures for 

 short-term experiments in large lakes. For studies of pollutant effects 

 on zooplankton, at least for short-term effects, the in situ method 

 used in this study has several advantages over other methods. Recent 

 work indicates that effects of enclosure in opaque polyethylene 

 carboys in longer experiments become excessive, but effects of 

 enclosure in translucent carboys incubated for 3 weeks at optimal 

 depths are not much greater than effects of large enclosures of 

 various kinds (Marshall and Mellinger, 1978a). Longer in situ 

 experiments still do not appear to be feasible in the Great Lakes, 

 however. Therefore, future determinations of relationships between 

 short- and long-term stress effects in laboratory populations of 

 species that live in these lakes (Marshall, 1978) and the same 

 relationships in zooplankton communities of small, experimental 

 lakes (Marshall and Mellinger, 1978b) wOl be useful in assessing the 

 long-term significance of the results of in situ experiments in large 

 lakes. 



ACKNOWLEDGMENTS 



The research reported here was performed under the auspices of 

 the U. S. Department of Energy. 



We thank J. A. Zischke and R. Van Reken for assistance in 

 various phases of the laboratory and field work. Helpful reviews of 

 an earlier version of this paper were provided by A. L. Buikema and 

 W. T. Edmondson. 



