4. Thermal Destratification 
During seasons when oxygen deficiencies exist 
in the bottom waters of a lake, nutrient avail- 
ability at the sediment-water interface is greatly 
increased. Oxygen deficiencies result largely from 
water stagnation in temperature layers and from 
biological decay on the lake bottom. Destratifica- 
tion or restoration of water circulation to allow 
oxygen to be absorbed from the air reduces the 
availability of nutrients in the bottom sediments, 
thereby arresting or retarding the aging process. 
Destratification can be accomplished by mech- 
anical mixing, aeration mixing, or thermal mixing. 
In the first, the lake is mechanically stirred so that 
zones of stratification are thoroughly mixed. In 
the second, the same mechanical mixing allows 
oxygen from the air to be absorbed in the water. 
In the third, mixing is accomplished by heating. 
5. Dredging 
Since bottom sediments are a potent source of 
nutrients, removal of the sediments has been 
recommended for restoring lakes. However, Great 
Lakes sediments are quite thick in certain areas. 
This and the expanse of the Lakes dictates 
exertion of Herculean efforts to completely 
remove the sediments. Care must be exercised to 
minimize release of nutrients during the dredging 
operation. 
6. Biological Control 
Biological control of algae and aquatic weeds is 
possible if suitable animal populations are discov- 
ered to graze on the blue-green algae and rooted 
vegetation. Developing strains of viruses or para- 
sites to prey exclusively on the algae and aquatic 
weeds is an alternative. Research to attain these 
objectives has had very little success to date. 
7. Chemical Control 
Although copper sulphate has been very 
successful for almost a century in controlling 
prolific growth of algae, it is also toxic to other 
life forms. Research investigators are seeking 
highly specific algacides to kill only the noxious 
species. Chemical control, not being a nutrient 
removal method, treats only the symptoms of 
eutrophication; the dead algae settle to the lake 
bottom, increasing the potential nutrient reservoir 
in the sediments. Further, it is only a temporary 
VI-128 
measure that must be repeated at frequent inter- 
vals, and the chemicals could accumulate in fish 
and eventually be harmful to man. 
8. Chemical Inactivation 
Research is in progress for a method to chem- 
ically inactivate the nutrients by preventing their 
utilization by the algae. One promising method is 
to develop chelating agents which will complex 
with divalent ions that function as co-enzymes in 
nitrogen fixation by algae and to determine the 
types of algae growths which result. 
9. Prevention of Light Penetration 
The development of a substance to decrease the 
penetration of light into the Lakes by increasing 
either reflectance or opacity has been proposed. 
This substance must be nontoxic, biologically 
stable, and nonrestrictive to oxygen transfer into 
the water. No known substance satisfies these 
criteria and others required to maintain maximum 
beneficial use of the water resource. Further, 
during certain seasons photosynthetic organisms 
can provide measureable quantities of oxygen 
during hours of sunlight, so the addition of a light 
retarding substance must not disrupt this. 
10. Rough Fish Removal 
Part of the nutrient inventory in the Great 
Lakes is retained by the fish population, and 
removing the fish reduces the inventory. However, 
many fish species are highly desirable for game or 
commerce. On the other hand, substantial popula- 
tions of rough fish, such as carp and alewives, are 
undesirable. 
A concerted effort to remove these fish would 
result in reduced nutrients. It would be prudent to 
consider processing these fish for usable protein, as 
burial of the fish within the Great Lakes Basin 
could allow the nutrients to be washed again into 
the lakes. 
Since desirable fish species are also a nutrient 
source, increased harvesting should be encouraged 
within the bounds of sound conservation practice. 
E. Other Measures for Water Quality Improve- 
ment 
Accelerated eutrophication, the cause of the 
most serious long-range consequences, is not the 
