It is believed that the observed differences in abundance in different 

 sections are in part the result of natural conditions, and in part the result 

 of pollution. In all probability, the increase of phytoplankton and organic 

 detritus resulting from pollution has made possible an increase of the Cru- 

 stacea. 



Bottom Organisms 



The criteria of pollution employed were as follows: A mud bottom having 

 less than 100 tubificid worms and more than 100 Hexagenia nymphs per square 

 meter was considered to be free from pollution; a larger number of tubificids 

 and smaller number of Hexagenia was regarded as evidence of pollution. Three 

 degrees of pollution were recognized, based on the number of tubificids per 

 aquare meter, as follows: light pollution, 100-999; moderate pollution, 

 1,000-5,000; heavy pollution, more than 5,000. On other than mud bottom, 

 only the tubificids were used as a criterion of pollution. 



In the Island Section quantitative sairples were taken only on mud 

 bottom. Nymphs of the burrowing mayfly, Hexagenia, were more abundant than 

 all other organisms combined. In 1929 the average number of Hexagenia for 

 seven stations was 283 per square meter, inhich was 65 per cent to the total 

 number of organisms. In 1930 the average number for five stations was 510 

 per square meter, which was 8? percent of the total. Considering only the 

 four stations sampled in both years, Hexagenia was about one and one-half 

 times as abundant in 1930 as in 1929. In both years most of the sampling 

 was done after the period of emergence of the insects. Very probably sampling 

 throughout the year would have shown much higher counts of Hexagenia. Tubi- 

 ficid worms were rare in both years. Areas with mud bottom in the Island 

 Section may be regarded as free from p o Hut ion by organic debris. Hauls of 

 the bottom sled in the shallower areas having hard bottom showed that these 

 also were not polluted. 



The average dry weight of Hexagenia nymphs for the two years was 143.2 

 kilograms per hectare (38.5 pounds per acre). This figure is close to that 

 for all organisms in a similar zone of Lake Mendota; it is below that of 

 Lake Wawasee, but above that of three other North Americai lakes. Thus, the 

 Island Section conpares favorably with inland lakes with respect to the 

 weight of bottom organisms per unit of area. 



There was no evidence of pollution of the bottom in the Portage River 

 Section near the mouth of the river. Definite evidence of pollution was 

 found near the mouths of the rivers in the Maumee Bay, River Raisin, and 

 Detroit River Sections. The estimated extait of the zones of heavy, 

 moderate, and light pollution for each section is shown in Figure 23, and 

 their areas are given in Table 100. The areas of the zones of pollution were 

 as follows: Heavy pollution, 25.2 square kilometers (9.7 square miles; mod- 

 erate pollution, U6.3 square kilometers (17.9 square miles); light pollution, 



