plankton has decreased. Secchi disc transparency, which had fallen from 

 3.7 in 19b0 to 1.0 in 1963, returned to 3.5 m in 1971. The lake is now no 

 longer considered eutrophic by many investigators. 



In the 1950's sufficient evidence had already come to hand to strongly 

 suggest that even huge bodies of fresh water such as the Great Lakes are 

 not immune to cultural eutrophication. Lake Erie, the shallowest and most 

 polluted of the five, was the first to exhibit serious deterioration. Sub- 

 sequently, similar changes in lake water quality were detected in southern 

 Lake Michigan, Lake Ontario, and other areas of the Great Lakes. Although 

 these lakes had been studied for many years from the point of view of 

 fisheries management, very little limnological work had been done prior to 

 the fifties. These evident eutrophication trends, however, served to 

 stimulated greatly increased research efforts at a number of United States 

 and Canadian universities, and by the governments of the two countries as 

 well. Great strides have since been made in furthering the knowledge of 

 these important lakes, leading to more intelligent management and utiliza- 

 tion of the resource. 



Also in the 1950' s , and indeed as far back as the 1940's, emphasis 

 began to shift from descriptive to experimental limnology, in which mani- 

 pulations are carried out on the full-scale or pilot-scale level, utilizing 

 lakes, ponds, or physical models. Such experimental methodology has become 

 fundamental to the development of lake-restoration techniques and proce- 

 dures, wherein results from the laboratory are carried to the field for 

 testing under controlled conditions. 



Studies of the nutrition of algae, both freshwater and marine, have 

 been going on since the 18th century. However, progress was slow compared 

 to other branches of biology, probably because of difficulties in culture 

 and manipulation. To define the nutritional requirements of algae, they 

 must be obtained in pure culture. As a consequence, progress in these 

 studies had to await the development of improved laboratory equipment that 

 would make this possible. As a result, during the 19th century algae were 

 studied almost exclusively from the morphological and taxonomic points of 

 view. By 1920 many species had been isolated in a bacteria-free state thus 

 setting the stage for investigations under controlled conditions. Today 

 the nutritional requirements of many species of algae are well documented. 



Through the first half of the century limnology was more or less 

 centered in the midwestern universities. Since World War II, however, 

 teaching and reseach programs in limnology, with related centers of 

 excellence in eutrophication, have been established in all the major geo- 

 graphic areas of the country. As a result, eutrophication problems pecu- 

 liar to various regions can now be considered much more thoroughly than 

 ever before. We hope these scientific resources will facilitate in many 

 ways the control of this problem through sound management decisions. One 

 such way is the continued synthesis of available information that bears on 

 eutrophication and control possibilities. 



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