LAKE EUTROPHICATION AND PRODUCTIVITY 



Figure VIII-17 — MEASUREMENTS OF ALGAE, PHOSPHORUS, 

 AND NITROGEN IN LAKE WASHINGTON 



150 



125 



100 



CD 



> 



Z 75 



50 — 



25 



1962 1963 



1964 



1965 1966 

 YEARS 



1967 1968 



1969 



The graph shows observations from 1962 to 1969 of the abundance of algae, 

 and the concentration of phosphates and nitrates. Yearly values are percentages 

 of the 1963 values of the concentrations. 



on an areal basis as kilograms per 

 hectare or pounds per acre. But this 

 is an imperfect and even misleading 

 calculation. Obviously, the effect of 

 a given input will vary with the 

 depth and volume of the lake. That 

 is, a given input will affect a shallow 

 lake more than a deep one. 



Also, this calculation takes no ac- 

 count of the fact that some of the 

 influents are much more concentrated 

 than others. It seems evident that 

 a very concentrated source relative 

 to lake water will mix in and raise 

 the nutrient content of the lake, 

 while a source with the same con- 

 centration will merely displace an 



equal volume and not make a net 

 increase. This is why sewage effluent 

 is so important relative to any normal 

 ground drainage: it can be 1,000 

 times as concentrated in phosphorus 

 as the natural water. One could 

 double the nutrient input of a lake 

 either by doubling the rate of inflow 

 at the same concentration or by 

 doubling the concentration in the 

 same inflow. The effects are likely 

 to be quite different. 



Thus, we have to learn how to 

 deal with the water budget as well 

 as the nutrient budget, and to cal- 

 culate the relative effect of influents 

 of different concentration. From the 



general viewpoint of basic "pure 

 science" limnology, this would im- 

 prove our understanding of the com- 

 parative limnology of productivity — 

 why lake districts tend to have a 

 certain uniformity and yet why in- 

 dividual lakes in one region differ 

 in productivity. 



Obviously, improvements in our 

 understanding of these matters would 

 be of tremendous practical impor- 

 tance. In few situations can there 

 be a clean, clear-cut, total diversion 

 of sewage as with Lake Washington. 

 Often one will want to know what 

 would be the effect of making some 

 percentage reduction in the phos- 

 phorus content of sewage, or of di- 

 verting some fraction of the effluent. 

 There is also the question of the 

 effectiveness of agricultural drainage, 

 which has probably been exaggerated. 

 Nevertheless, we could find out 

 whether it would be worthwhile try- 

 ing to modify agricultural practices 

 in a watershed. 



Conversely, one might want to 

 make a very precise adjustment of 

 enrichment to maintain fish produc- 

 tion at as high a level as possible 

 without damaging the desired species 

 or creating algal nuisances. 



The problem, then, is to improve 

 the predictability of limnological con- 

 ditions, especially the productivity 

 and abundance of organisms. 



Demonstrably, Lake Washington 

 has permitted a step forward in this 

 direction, but we have a long way 

 to go. Progress is more likely to be 

 made by limnologists working with 

 very broad questions than by any- 

 body else working with very specific 

 and limited questions on a purely 

 practical basis. Progress will be 

 faster and better if some more experi- 

 ments can be carried out with real 

 lakes. For example, Lake Erie might 

 be regarded as a prime object for 

 limnological experimentation. 



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