distance from the river. There was little difference in abundance of 

 diatoms at the three stations. It should be noted that diatoms were 

 less abundant than greens or blue-greens, particularly at the most 

 heavily polluted station, and that blue-greens were more abundant than 

 greens at each station. The dominance of the blue-greens can be ex- 

 plained by the abundance of nutritive material, for it is well knoim 

 that this group is particularly favored by such a condition. 



Another point worthy of notice is that the phytoplankton at 

 Station 2^0, 2^2, and 2$U was more abundant in 1930 than in 1929. 

 The cause of this difference is not known with certainty, but it 

 appears probable that the cause was a difference in concentration of 

 nutritive materials in the two years, which in turn was dependent upon 

 a difference in discharge of Maumee River. It was pointed out on 

 page 86 that in the months of July, August, and September, 1929, the 

 mean discharge of the river was 21^9 cubic feet per second, and that 

 the corresponding figiu^e for 1930 was only 23U cubic feet per second. 

 If the amount of sewage entering the river remained the same, the 

 average concentration of nutritive materials in the lower river would 

 have been greater in 1930 than in 1929* -Moreover, there would be 

 less outflow from Maumee Bay in 1930, and hence a greater concentration 

 in the bay. 



The great abundance of plankton in this area indicates that 

 poisonous trade wastes were not present in sufficient quantities to 

 kill plankton organisms. 



In addition to the large numbers of living algae, the water 

 of this area contains much non-liidng matter of organic nature. 

 Probably sewage is the most important source of this material. Some of 

 it is so finely divided that it remains in suspension a long time. 

 Waves and passing vessels tend to keep the water in motion and prevent 

 settling out. In the centrifuge plankton sa-nples the minute particles 

 were quite uniformly distributed through the liquid so that estimation 

 of the amount present was impracticable. However, this organic 'detritus 

 must be an important item of food for the rotifers and Crustacea of the 

 plankton, and should be taken into account in a general way. It was 

 much more conspicuous in this section of the lake than in any other. 



River Raisin Section 



The River Raisin Section is represented by only one station, 

 Station 117, about two miles out from the shore, vrfiere the water is six 

 meters deep. The data collected here on five dates in 1929, and on 

 eight dates in 1930 are given in Table 59« 



189 



