Reference to Table 83 shows that the nymphs of the burrowing 

 mayfly, Hexagenia, were far more abundant than any other kind of 

 organism. At six of the seven stations the number exceeded 200 per 

 square meter, and at Station 37A (Kellys Island) reached the high num- 

 ber of 5o8. At Station 75 (West Sister Island) there were only h3 per 

 square meter. The reason for their comparative rarity at this station 

 is not known, but obviously it was not pollution, for tubificid worms 

 were rare also. It may be seen that the mean number of 283 per square 

 meter for all seven stations was about five times as great as the 

 number of the next most abundant group, the Chironomidae. The remain- 

 ing groups were rather rare at most of the stations and none reached a 

 mean number of 25 per square meter, or one per haul of the dredge. The 

 tubificid worms ranged in number from 2 to 22 per square meter, and the 

 mean for the seven stations was twelve. 



In 1930 samples were taken at only five stations; Station 82 

 was abandoned and Station 72 was substituted for Stations 68 and 75. 

 Fewer samples were taken also: U3 in 1930 as compared with 121 in 1929. 

 It should be remembered, however, that the dredge used in 1930 covered 

 almost twice the area of the one used in 1929> so that the difference 

 in area dredged was much less than the difference in number of samples 

 indicates. 



The data in Table 8I4 show that in 1930 Hexagenia was much more 

 abundant than in 1929. Of the four stations studied in both years, 

 only one (Station 37a) showed a lower count for 1930, and the difference 

 was small compared with the increase at the other stations. If we assume 

 that the data collected at the four stations were representative of con- 

 ditions in the two years, we may say that Hexagenia was about one and 

 one-half times as abundant on mud bottom in 1930 as in 1929, The mean 

 number for the two seasons (all stations) was 396 per square meter, 



^11 of the other groups of organisms were less abundaijt in 

 1930 than in 1929 except the snail, Amnicola. The mean number of organisms 

 other than Hexagenia for all stations was l53 per square meter in 1929 and 

 only 77 per square meter in 1930, Thus, there was a marked increase in 

 the numbers of Hexagenia relative to the other organisms. In 1929 

 Hexagenia made up 65 per cent of the total number of organisms, while in 

 1930 it made up 87 per cent of the total. 



These percentages are probably much lower than they would have 

 been had the sampling been carried on over the entire year. Most of the 

 sampling was done following the emergence of tremendous numbers of 

 Hexagenia in late June and early July. The effect of this emergence is 



260 



