creasing pressure on the Great Lakes as a recrea- 
tional resource. The change is compounded by the 
fact that as population density increases, social 
pressures arise to place a higher value on recrea- 
tional opportunities. Therefore, the value to the 
individual of such opportunities in the future will 
be greater than today. By the year 2020, for 
example, the projected population in the Lake 
Erie Basin alone is expected to pass 23 million, 
compared to 9.8 million in 1960. The rate of 
increase in water-based recreation demand is esti- 
mated to far exceed this population growth. 
There has been much speculation about the 
cost of a restoration plan for Lake Erie. The 
following example provides an order-of-magnitude 
estimate of cost for a potentially feasible method. 
It already has been shown that preventive meas- 
ures must be implemented to remove the causes of 
accelerated eutrophication. Moreover, it has been 
stated that restorative measures must be imple- 
mented so that the time of lake recovery will be 
short enough to avoid forfeiture of substantial 
benefits. The example assumes that the recommen- 
dations for preventive technology will be imple- 
mented. 
Earlier subsections on Prevention and Restora- 
tion (C and D) outlined the technology considered 
and, in some cases, tested on a small scale. The 
following is one approach involving a large engi- 
neering effort. It is a method to destratify Lake 
Erie by artificial recirculation. This example is not 
offered as the ideal solution. (Undoubtedly a 
complex of restoration methods will be required, 
and several techniques must be implemented.) It is 
intended to provide a reference concept that will 
assist in defining the necessary level of planning, 
effort, and funding in a more quantitative manner 
than previously. As such, the artificial recircula- 
tion case study can serve as a focus for further 
evaluation. 
It must be emphasized strongly that restoration 
of a lake as large as Erie represents a major 
environmental modification and, hence, must be 
approached with caution. The analysis and evalua- 
tion required before such an undertaking is 
beyond the scope of this discussion. Although 
much information necessary to evaluate the feasi- 
bility, engineering requirements, and effects of an 
artificial recirculation project already exists, sub- 
stantial additional work will be required. 
As other restoration plans proposed earlier, this 
example is directed toward reducing the avail- 
ability of nutrients from bottom sediments. It has 
been demonstrated experimentally that the avail- 
ability of nutrients to the water mass is regulated 
by the oxygen content of a lake’s deep waters. 
As these waters become oxygen deficient, decom- 
position of organic matter without oxygen results 
in chemically reducing conditions in the sedi- 
ments. This further results in iron and manganese 
compounds being dissolved and sediment nutrients 
being released. Further study of this phenomenon 
is an integral part of the National Eutrophication 
Research Program wherein pilot scale tests in 
Klamath Lake, Oregon, will be used to determine 
the influence of sediment-water interchange on 
algae production. 
A second effect of oxygen depletion in the 
deep waters is the marked change in the aquatic 
ecology. The demise of the mayfly nymph on the 
bottom of Lake Erie is related to the low oxygen 
concentration. Since the mayfly provided a major 
source of food for desirable fish species, these too 
have declined. A more direct effect of low oxygen 
concentrations is the gradual takeover of the Lakes 
by rough fish that are more tolerant of low oxygen 
levels. 
In the Public Health Service 1965 report, 
Pollution of Lake Erie and Its Tributaries, the 
dissolved oxygen values in the bottom waters of 
the central basin of Lake Erie are described as 
having decreased during the past 35 years from 
about five milligrams per liter (mg/l) to less than 
two, with many areas near zero. Typically, severe 
late summer stratification can occur over an area 
of about 2,600 square miles or about 25 per cent 
of the entire lake. The total volume of the water 
column (average depth—12 fathoms) in this area is 
about 120 million acre feet or 35 cubic miles. 
Carr! ? provides an excellent review of dissolved 
oxygen conditions in Lake Erie. Although it is 
difficult to generalize, Carr’s work suggests that 
the oxygen-depleted hypolimnion occurs in the 
water column below 50 feet. For the particular 
configuration of Lake Erie, this condition prevails 
for about the last 10 feet of the central basin. If 
'3carr, J. S., Dissolved Oxygen in Lake Erie, Past and 
Present, University of Michigan Great Lakes Research 
Division, Pub. 9, pp. 1-14. 
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