of hard clay and no adequate sample could be taken. At Station 221B 

 there were 702 Tubificidae per square meter and Hexagenia larvae 

 were absent. Obviously a separate investigation would be necessary to 

 determine the distribution of organisms in this general area. 



The Tubificidae were most abundant at Station 222, located 

 at Detroit River Lit-ht, about three miles out from the mouth of the 

 river as defined in this report. The count of 1$00 worms at this 

 point indicates moderate pollution. But at Station 126, two miles 

 below, the counts of 1929 and 1930 show that the bottom was only 

 lightly polluted. In spite of the apparent inadequacy of the data at 

 Station 222 and in the lower part of the river, it seems reasonable to 

 assign this are to the zone of moderate pollution and to draw the line 

 of demarcation (between it and the zone of light pollution) between 

 Stations 222 and 126. 



Sand bottom was encountered at Station J.27, probably as a 

 result of dredging operations at the time the down-bound channel 

 was extended southward. Only a few tubificids were taken. At 

 Station 128, one mile farther out, the bottom was composed of mud 

 and the tubificid count was again in the range of light pollution and 

 there were a few Hexagenia larvae. At Station 130, located two miles 

 from the preceding station, the mayfly larvae were still few in 

 number in spite of the very low tubificid count. In accordance with 

 the policy of using only the Tubificidae in case of conflicting evi- 

 dence, the line separating the zone of light pollution from the zone 

 of clean bottom would be placed between Stations 128 and 130, At 

 Stations 132 and 13li the bottom was obviously free from pollution. A 

 single Scimple taken at Station 131 (half way between Stations 130 and 

 132) in 1928 yielded U30 Hexagenia per square meter. 



In order to determine the extent of pollution to the west and 

 northeast of Station 126, six special stations (239, 2ii0, 2hl; 223, 22li, 

 and 22$) were established as indicated in Figure 23. The data obtained 

 are shown in Table 98. Only one of the six stations had bottom soft 

 enough to yield a sample which could be regarded as quantitative. This 

 was Station 2^0, located 2 miles west of Station 126. Here there were 

 122 Tubificidae per square meter but no Hexagenia, indicating very 

 light pollution. In view of the negative results obtained at Stations 2Ul 

 and 239, ^^d at other nearby stations (Table 95). it seems advisable to 

 place the line of demarcation between Stations 2U0 and 2Ul, It must be 

 admitted, however, that further collecting might require a change in its 

 position. Toward the northeast from Station 126 it was found impossible 

 to take a good sample of the bottom, but there was no evidence of organic 

 debris, 



295 



