samples from muddy bottom in the Island Section, far from sources of 

 pollution, showed that the nymph of the burrowing mayfly, Hexagenia, 

 was by far the most abundant organism, while tubificid worms of the 

 genera Tubifex and Limnodrilus were found only occasionally, and never 

 in large numbers. Samples taken at the mouth of Maumee River contained 

 prodigious numbers of Tubificidae but no Hexagenia. Here there was a 

 great deal of organic matter, but in the Island Section the mud was 

 notably free from it. Samples taken h and 8 miles out from the river 

 showed a progressive decrease in the organic matter and Tubificidae, 

 and the appearance of Hexagenia. Similar experiences in other parts of 

 the lake finally led to the adoption of these two kinds of organisms 

 as the index organisms for those parts of the lake having a mud 

 bottom. In this connection it is interesting to note that Richardson 

 classified Tubifex and Limnodrilus as pollutional forms, and Hexagenia 

 as a clean water form, Hexagenia prefers a soft mud bottom, and it finds 

 conditions on hard bottom unsuited to its mode of life. For that reason 

 it loses its index value on hard bottom, such as sand or hard clay. 

 Tubificidae, however, thrive on a hard bottom provided there is a super- 

 imposed layer of decaying organic matter. The presence of large numbers 

 of tubificids is not always an indication of sewage pollution. Popula- 

 tions of several thousand per square meter have been reported from the 

 profundal region of Lake Mendota (Juday, 1922, p. L86) j and of Third 

 Sister Lake (Eggleton, 1931, p. 279) . In these two cases the organic 

 matter which supported the large populations was derived from natural 

 sources. 



The general plan of investigation, as suggested in the pre- 

 ceding paragraph, was to determine conditions in the lake far from the 

 mouths of rivers, and then to compare them with conditions at the mouths 

 of rivers where pollution would be suspected. This method was adequate 

 to indicate the presence of pollution at the mouths of the rivers, but 

 the regular stations were too few in number to permit determination 

 of the extent of the polluted areas. In order to get sufficient data 

 to make possible the attainment of this objective, a large number of 

 special stations were established. These special stations were estab- 

 lished along lines running from polluted stations toward the shore or 

 toward points in the lake which were known not to be polluted. The 

 distance between stations was determined largely by local conditions; 

 in some area they were placed l/ii mile apart and in others as much as 

 two miles apart. All of the work of this kind was done in the summer 

 of 1930, and because of the limitations imposed b^ the large area under 

 investigation and the small amount of time available, fewer samples were 

 taken than was desirable. However, it is believed that the lijnits of 

 the polluted areas were determined with a fair degree of accuracy, 



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