hy the content of albuminoid ammonia. It is quite likely that the 

 sections now relatively rich in plankton were also relatively rich 

 under natural conditions. Yet there is little doubt that the algae 

 of the plankton have increased to an important degree as the result 

 of the additional nutritive material derived from domestic sewage. 



The algae of the plankton perhaps are used very little as 

 food for fishes, directly, but indirectly, they are important as food 

 for Crustacea and rotifers. The relative positions of the sections 

 with respect to the abimdance of Crustacea of the plankton was the 

 same as with respect to the abundance of phytoplankton. It is almost 

 certain that the increase of phytoplankton (and of particulate, 

 non-living, organic matter derived from sewage) has made possible an 

 increase in the abundance of Crustacea. 



This increase in Crustacea may be regarded as advantageous 

 to the fishery, for it should permit more young fish to find an 

 adequate supply of food, and thus to escape one of the hazards of 

 post-larval life. Also it should permit the lake to maintain a larger 

 population of adult individuals of plankton-feeding species. The 

 advantage is not entirely restricted to the fishes which d epend 

 directly upon the plankton. Many bottom invertebrates subsist largely 

 on detritus derived from dead plankton organisms, and it is reasonable 

 to suppose that they have increased as a result of the additional food 

 available to them. This increase should be passed along to the fishes 

 of bottom-feeding habit. Briefly, the trophic standard of the lake 

 has been raised by pollution, and the ability of the lake (from a 

 nutritional point of view) to support fishes has been enhanced cor- 

 respondingly. 



Conclusion 



As stated before, it is not possible to evaluate the harmful 

 and helpful effects of pollution in numerical terms, to determine the 

 total or residual effect on the fishery, .Vhether the residual effect 

 of pollution has been to increase or decrease the productive capacity 

 of the lake is open to question. Clearly pollution has not been an 

 unmixed evil, and there is some basis for the view that it has done 

 more good than harm. Even though the residual effect may have been 

 detrimental, it seems highly improbable that pollution in the western 

 part of the lake has been the primary or controlling factor in the 

 depletion of the fishery of Lake Erie, 



310 



