PART VIII — AQUATIC ECOSYSTEMS 



they would respond just as sensi- 

 tively. These considerations are rel- 

 evant to making predictions and to 

 the development of plans for han- 

 dling the pollution of the Great Lakes 

 and others that are receiving effluent 

 or are going to. 



If one is going to make a complete 

 statement about the processes that 

 control the productivity and abun- 

 dance of organisms in lakes, he has 

 to refer to the whole set of environ- 

 mental variables: nutrients including 

 (in addition to nitrogen and phos- 

 phorus) carbon, such micronutrients 

 as iron, copper, cobalt, and others. 

 He has to discuss light penetration 

 into lakes, the kinetics of photosyn- 

 thesis, and a great many other things. 

 But to take practical steps to improve 

 the condition of a particular lake that 

 has been polluted is another matter. 

 It is not necessary to recapitulate the 

 entire history of limnological investi- 

 gation in that lake. We know a great 

 deal already, and can make use of 

 the general knowledge we have de- 

 veloped from pure limnological re- 

 search. 



A point of particular interest has 

 to do with the relative importance 

 of phosphorus (P) and nitrogen (Nj, 

 a matter about which there has been 

 some uncertainty. For practical con- 

 trol, the proposition very often would 

 be to ask what the effect would be 

 of removing most of the phosphorus 

 from the effluent. That is, to simplify 

 a little, what would be the effect of 

 heavily enriching with nitrogen? The 

 answer to this has to lie in the condi- 

 tion of the receiving water. If the 

 natural waters, for geological reasons, 

 are relatively rich in N, so that P is 

 the primary limiting factor, phos- 

 phorus enrichment is likely to in- 

 crease production. 



The obvious thing to do is to find 

 out which element is limiting in 

 each particular case. A variety of 

 techniques exist, ranging from bio- 

 assay with lake-water samples to 

 which nutrients are added through 

 analysis of lake water and plankton 



for N and P. Some studies have 

 shown that added P alone was enough 

 or almost enough to account for the 

 observed effect of sewage. 



Another rather new approach to 

 this problem of diagnosis shows that 

 in Lake Washington, before pollu- 

 tion, nitrate was in excess in the sense 

 that when phosphate approached zero 

 during the spring growth of phyto- 

 plankton, there was a distinct excess 

 of nitrogen. After pollution with 

 phosphorus-rich sewage, P was in 

 excess in 1962. In 1962, then, Lake 

 Washington might well have re- 

 sponded to an increase in nitrogen 

 which would have permitted the 

 phosphorus to be used up. 



This point is easy to get mixed up. 

 One must keep clear whether one 

 is talking about the effect of adding 

 an element or removing it. One does 

 the first when trying to explain why 

 a given lake has gone into nuisance 

 conditions; one does the latter when 

 thinking about how to improve the 

 situation by removing something. 

 In June of 1962, adding phosphorus 

 to Lake Washington would not have 

 increased algae because there was 

 an excess. Removing phosphorus 

 would have decreased algae. 



In the long run P is the more im- 

 portant element in much of the world. 

 But there are places where phos- 

 phorus is relatively rich in the nat- 

 ural water supplies. Goldman has 

 proposed, for instance, that Lake 

 Tahoe would be susceptive to ni- 

 trogen enrichment. Also, there are 

 some organisms that seem able to 

 get along with much less phosphorus 

 than others. 



If all this is correct, then for each 

 case we have to identify a key ele- 

 ment, limitation of which would im- 

 prove the lake. One could theoreti- 

 cally limit production by eliminating 

 any essential element, but in fact 

 there are very few elements that one 

 can control. The single element that 

 is most easily removed from effluents 

 is phosphorus. So the question boils 



down to asking whether limiting 

 phosphorus, either by removal from 

 sewage or by limiting detergents, will 

 be enough to make an adequate dif- 

 ference. This means, too, that one 

 must find out whether there is enough 

 P from agricultural drainage into a 

 given lake to make a difference. 



In Lake Washington, phosphorus 

 has decreased much more than nitro- 

 gen or carbon dioxide. (Sewage is 

 relatively much richer in phosphorus 

 than is the natural water supply to 

 the lake.) The abundance of algae 

 has decreased in very close relation 

 to phosphorus, not in relation to 

 nitrogen. (See Figure VIII— 17) This 

 suggests that, in similar lakes, any 

 limitation on the amount of concen- 

 trated sources of phosphorus reaching 

 the lake will be beneficial. That is, 

 improvement should result in propor- 

 tion to the removal of concentrated 

 sources of phosphorus. Large changes 

 could be made by reducing the phos- 

 phorus content of detergents. In 

 some places it may be worthwhile to 

 install treatment processes to remove 

 phosphorus from effluent. 



This discussion is focused on the 

 eutrophication problem. Inflow of 

 toxic wastes (lead, mercury, cyanide, 

 herbicides, DDT, etc.) was not an 

 important part of the Lake Washing- 

 ton problem, but it evidently is a part 

 of the Lake Erie problem. In every 

 case of lake deterioration, one should 

 find out if toxic wastes are important. 

 Nevertheless, Lake Erie would prob- 

 ably be measurably improved by lim- 

 itation of sewage phosphorus. Char- 

 acterizing Lake Erie as "dead" seems 

 quite incorrect. 



Unsolved Problems 



Plenty of interesting problems re- 

 main in studying the natural mech- 

 anism of control of productivity and 

 abundance of organisms in lakes. 

 In particular, what is the relation 

 between the rate of input of nutrients 

 and the productivity of lakes? It is 

 common to express the annual input 



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