originate in Saginaw Bay under certain conditions can survive transport into 

 the extreme southern part of Lake Huron, It is difficult to imagine this 

 occurring unless these populations were growing fast enough to replace 

 grazing and sinking losses. The other plausible effect is that death of 

 these populations, through grazing or other process, will release additional 

 phosphorus and thus stimulate eutrophication of the offshore waters of Lake 

 Huron. To our knowledge this type of biological loading has not been con- 

 sidered in the limnological literature, but it may be an important mechanism 

 of pollutant dispersal in the Laurentian Great Lakes. 



Our data also suggest that incorporation of Pb in polyphosphate bodies 

 may be an important mechanism for dispersal of the toxicant in aquatic 

 systems. To our knowledge, our report of the polyphosphate-lead association 

 is the first demonstration of this mechanism in naturally occurring popula- 

 tions. The fact that this type of uptake can be produced in the laboratory 

 conditions and Crang and Jensen's (1975) demonstration of titanium incor- 

 poration in polyphosphate bodies in Anacystis nidulans Dr. and Daily sug- 

 gests that binding of heavy metals in osmotically inert inclusions such as 

 polyphosphate bodies could be a general mechanism for protecting phytoplank- 

 ton cells (at least temporarily) against heavy metal toxicity. Our results 

 to date suggest that this is probably not the case. Our experiments with 

 metals more directly toxic to algae, such as Cu and Cd, as well as Zn show 

 that although stress induced by the presence of these elements at relatively 

 low levels may induce polyphosphate body formation, these elements are not 

 sequestered in the polyphosphate bodies to any measurable extent. This 

 situation should be further investigated as it is possible that organisms 

 other than those so far investigated may be able to affect heavy metal in- 

 corporation in polyphosphate bodies or that incorporation may take place at 

 concentrations other than those tested. 



Our results are also interesting in respect to previous reports of heavy 

 metal accumulation in algae. Silverberg (1975) demonstrated that Pb accumu- 

 lated in the cell wall and in the peripheral vacuole of Stigeoclonium tenue 

 (Ag.) Kutz. Silverberg (1976) also found that exposure of 3 species of 

 green algae to relatively high levels of Cd resulted in degenerative changes 

 in the mitochondria of the cells and the formation of granules within the 

 mitochondria which apparently contained Cd. Although we have observed some 

 changes in cellular organelle structure in our experiments, we have not ob- 

 served measurable accumulation of Cd or Zn associated with any organelle of 

 specific cellular site. It should be noted that the concentrations used in 

 Silverberg's experiments were 3 to 10 times higher than the concentrations 

 tested in our experiments. It is probable that the cellular modifications 

 he noted are symptomatic of acute toxicity. 



At this stage of our investigations many questions remain to be an- 

 swered. We are, none the less, encouraged in that the application of modern 

 instrumentation and techniques has provided some insight to the complex 

 interactions of nutrient and heavy metal contamination in large aquatic 

 systems. It is clear that an understanding of cellular level processes is 

 essential to understanding system level processes and the development of 

 effective management strategies. In the particular case of the Saginaw Bay 

 pollution problem application of these techniques has elucidated a mechanism 



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