The Island Section of Western Lake Erie is richer in plankton than Lake 

 Eirie east of that area, and richer than Lake St. Clair. Comparisons with 

 Lake Mendota, a eutrophic lake, and Green Lake, an oligotrophic lake, on the 

 basis of the dry weight of organic matter in the centrifuge plankton in 

 autumn (and other considerations), show that Western Lake Erie stands between 

 the two in richness. It probably stands nearer to Lake Mendota than to Green 

 Lake. Since these two lakes are fairly typical of their classes, and since 

 eutrophic lakes are generally rich and oliogotrophic lakes generally poor, 

 the Island Section of Western Lake Erie might be described as "moderately rich" 

 in plankton. 



Large and highly consistent inequalities in horizontal distribution exist 

 in Western Lake Erie as a whole. For the months of July, August, and Septem- 

 ber of 1930, the average abundance per unit volume of water in the Detroit 

 River Section was l/U of that is the Island Section; l/ll of that in the 

 Portage River Sectionj I/16 of that in the River Raisin Section; and 1/26 of 

 that in the Mauraee Bay Section. The data do not permit such a definite state- 

 ment of relative abiindance for 1929. As far as comparisons can be made, they 

 indicate that the relative positions of the sections were the same in both 

 years (with one minor exception) but that differences in abundance were not 

 as marked as in 1930. The algae were distinctly more abundant in Maumee Bay 

 and River Raisin Sections in 1930 than in 1929. Qualitatively, the s ections 

 having the most abundant plankton w ere characterized by the dominance of 

 blue-greens over greens and diatoms. 



The most probable explanation of the differences in abundance between 

 sections is as follows. The sections which are now especially abundant in 

 plankton (Maumee Bay, River Raisin, Portage River Sections) were abundant in 

 plankton under natural conditions. Shallowness of the water is believed to 

 have been the principal contributing factor in this richness, with the added 

 factor, in the case of the Portage River Section, of the lacustrine character 

 of the lower river. Superimposed upon this natural richness is the richness 

 caused by the nutritive salts derived from domestic sewage. Detroit River 

 Section is poor in plankton because the source of the river, Lake St. Clair, 

 is poor in plankton, and not because of the d estructive effect of poisonous 

 chemicals derived from industrial wastes. There is little or no local 

 increase of abundance resulting from domestic pollution in this section. The 

 natural abundance of plankton in the Island Section has been increased as a 

 result of pollution, by the eastward drift of organisms produced near the 

 rivers, and by the use of the excess of nutritive salts. The relative posi- 

 tions of the different sections of the lake with respect to abundance of phyto- 

 plankton was the same as with respect to intensity of pollution as indicated 

 by the content of albuminoid ammonia. 



Zooplankton 



The Crustacea were not uniformly distributed in the Island Section, 

 but there is no evidence that they were consistently'' abundant at certain 



