reduction in averaf^e number from 383,^00 per square meter in 1929 

 to onXy l$,h70 in 1930 is readily explained by the changes in the 

 bottom noted above. There was also a notable decrease in the 

 number of Musculium transversum , a sphaeriid regarded by Richardson 

 (1928)as lonusuaLly tolerant to pollution. The many young Sphaeriidae. 

 which were not identified to genera in 1930, were probably Musculium, 

 as in 1929. The chironomidae showed an increase in 1930 over 1929, 

 but since the identifications were not carried beyond the family 

 group, the significance with regard to pollution is open to question, 

 A slight indication of better conditions in 1930 is seen in the 

 appearance of small numbers of Sphaerium, and Bithynia, both of which 

 are regarded as being much less tolerant to pollution than 

 Musculium transversum . Hexagenia nymphs were not found in either year, 

 nor were the molluscs, Valvata, Amnicola, and Pisidium, 



This station must be re[,arded as heavily polluted. Examina- 

 tion of the bottom deposits shows the presence of a large amount of 

 decaying organic matter, and the most abundant organisms are tubificid 

 worms and Musculium transversum , which are rare in the open lake and 

 are known to thrive in polluted areas. The case is strengthened by the 

 total absence of Hexagenia, a clean-water form which is characteristic 

 of the open water, far from sources of pollution. The apparent im- 

 provement noted in 1930 was not a real improvement at all, because the 

 objectionable deposits were merely transferred from one place to another. 



No attempt was made to weigh the organisms taken at 

 Station 250. However, in view of the tremendous numbers of Tubificidae 

 observed there in 1929, it may be of interest to estimate their weight 

 per unit area. Juday (1922, p. I486) found the average dry weight of. an 

 individual tubificid to be 0,3119 milligram. If the tubificids at 

 Station 250 had the same average weight, the dry weight of these forms 

 alone was 1,196 kilograms per hectare (1,06? pounds per acre) in 1929. 

 This figure is roughly 39 times as great as the corresponding figure 

 for Hexagenia in the Island Section for the same year. It is evident 

 that the heavily polluted bottom supported a much larger crop of living 

 organisms than the clean bottom of the open lake, 



Wiebe (1928, p. IU8) reported one station in the upper part 

 of Mississippi River which had a somewhat denser population of tubificids 

 than Station 2^0 in 1929. The station was within the city limits of 

 Minneapolis and yielded 36[i,000 worms per square yard, or 3l47,000 per 

 square meter, Wiebe found a number of points where there were several 



270 



