An excellent beginning at such a synthesis occurred at the Inter- 

 national Symposium on Eutrophication sponsored by the National Academy of 

 Sciences and held on the campus of the University of Wisconsin in 1967. 

 Proceedings of this symposium were published under the title "Eutrophica- 

 tion: Causes, Consequences, Correctives" (National Academy of Sciences, 

 1969). This went far to bring together much of the existing knowledge of 

 eutrophication processes and controls. A second review document, "Eutro- 

 phication--A Review," was published in 1967 (Stewart and Rohlich, 1967). 

 Together, these two publications emphasized the state of the art, pointing 

 the way for future work. Their appearance at this critical time marks the 

 year 1967 as an especially significant milestone in eutrophication and lake- 

 restoration research. 



THE LAST EIGHT YEARS 



The period from 1850 to 1967 provides the historical preclude to the 

 modern reaction to the eutrophication problem in the United State. No 

 doubt, the manpower and dollars spent in the past 8 years to understand and 

 cope with eutrophication far exceeds those spent in all of the preceding 

 117 years. The outlook today is that expenditure to activate remedial 

 technology soon will be greater than dollars spent to develop new techni- 

 ques. 



Two factors lead to this conclusion: (a) several remedial approaches 

 are available, and (b) the number of lakes to be addressed is very great. 

 In the United States, excluding Alaska, there are about 100,000 lakes. At 

 best, this is only an estimate because no standard definition of a lake is 

 used by all states 1 . It is estimated, also, that 12,000-15,000 lakes are 

 over 4.5 ha and that 10-20% are eutrophic. Generally, lakes in or near 

 urban development are eutrophic, whereas many of those in relatively 

 sparsely populated areas are more likely to be oligotrophic. 



Today, one can still ask: What causes eutrophication? Many inter- 

 acting factors contribute to the overall process. Productivity depends on 

 solar radiation, temperature, lake-basin morphology, water-retention time, 

 and perhaps most important, the availability of adequate nutrients. It is 

 generally agreed that algae and higher aquatic plants require 25-30 differ- 

 ent nutrients for growth. Large amounts of carbon, nitrogen, hydrogen, and 

 phosphorus and smaller amounts of approximately 25 others, such as magne- 

 sium, calcium, boron, zinc, copper, molybdenum, and manganese, are 

 necessary. In addition, vitamins such as B 12 , thiamine, and biotin, and 

 hormones play a part in nutrition. In theory, since all of the above are 

 essentially for growth, the unavailability of any one could control eutro- 

 phication. Generally, however, nitrogen and phosphorus emerge as the criti- 

 cal elements in controlling aquatic plant nuisances. 



Some states report, as lakes, bodies of water over 1.21 ha (3 acres); 

 Others, over 4.05 ha (10 acres); and others over 40.5 ha (100 acres). 



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