The U.S. Environmental Protection Agency (EPA) initiated "The National 

 Lake Survey" in the summer of 1972. This project determines the location, 

 severity, and extent of eutrophication in lakes and impoundments that act 

 as receiving waters for municipal waste-treatment-plant effluents. At a 

 cost of $7-8 million and a time span of at least 4 years, the study will 

 examine 812 major lakes and impoundments in 48 states. For each lake the 

 trophic state is estimated, and the sources and magnitudes of nitrogen and 

 phosphorus supplies are identified, to judge whether reduction in phos- 

 phorus loading will restore or protect the lake. 



To sample the lakes and measure limnological characteristics, EPA uses 

 three helicopters equipped with special remote and contact sensors. Each 

 lake is sampled, at multiple sites, three times during the growing season. 

 The pontoon-equipped helicopters land on the lakes where probes are lowered 

 into the water to measure dissolved oxygen, conductivity, temperature, and 

 turbidity at different depths. Samples from various depths are analyzed 

 for approximately 15 parameters. Algae are identified and counted. Algal 

 assays are conducted on the lake waters to determine the productivity poten- 

 tial of the water and to assist in the identification of the limiting nutri- 

 ent. The major streams entering or leaving the lakes are sampled for nitro- 

 gen and phosphorus. Stream flows are measured, and sewage treatment plant 

 effluents are also sampled. 



Another phase of the survey's work is to develop techniques to deter- 

 mine relationships between land-use patterns and nitrogen and phosphorus 

 supply by using aerial photography. Land-use types will be correlated with 

 the trophic condition of lakes and refined nutrient flux factors developed 

 for various land-use types and geographic areas. This and other informa- 

 tion from the survey has made it possible to focus on the following aspects: 



1) Relationships between drainage-area characteristics and 

 non-point source nutrients in streams (Figures 1 and 2); 



2) quantitites of nitrogen and phosphorus in wastewater 

 effluents; 



3) the relationships of phosphorus and nitrogen to the trophic 

 state of northeast and north-central lakes and reservoirs 

 (Figures 3 and 4); and 



4) an approach to a relative trophic index system for classi- 

 fying lakes and reservoirs. 



The final step of the survey will be the interpretation of the data for 

 each lake and, in cooperation with appropriate state agencies, the deriva- 

 tion of subsequent recommendations for remedial action. 



Algal Assays 



Algal assays have been for some time an extremely valuable tool for 

 evaluating water quality in relation to eutrophication. Many investigators 



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