River. Moreover the different two-week periods show little change 

 from that order. With the exception of two periods when the 

 Portage River Section showed somewhat higher counts than the River 

 Raisin Section, the order is the same as in the mean for all periods. 



If the mean abundance in the Detroit River be assigned a 

 value of one, the relative abundance in the other sections would be 

 as follows: Mauraee Bay, 26; River Raisin, l6; Portage River, 11; 

 Island, li. In other words, for the period under consideration, 

 Maumee Bay contained 26 times as much phytoplankton as the area near 

 the mouth of Detroit River; and similarly for the other sections. 

 Inspection of the less regular and complete data for 1929 suggests 

 that the order of the sections was the same as in 1930, except 

 possibly that the Portage River Section would displace the River Raisin 

 Section from the second position. However, it seems probable that the 

 disparity between sections was less marked in 1929 than in 1930, 



It is believed that the large and consistent differences 

 observed in 1930 were due, in part, to natural conditions. As stated 

 before, Maumee Bay and the lake nearby probably were very productive 

 before man changed some of the physical and chemical conditions in that 

 area, and there is little doubt that Detroit River is naturally poor 

 in plankton. With the growth of large cities on tributaries of the 

 lake, an immense amount of sewage, containing various forms of nitrogen 

 and other compounds useful to plants, has been added to the water. 

 The effect which this added food supply has had on the plants of the 

 plankton cannot be determined quantitatively, but that the effect has 

 been in the nature of an increase in abundance is hardly open to 

 question. 



It appears probable that the increases owing to pollution have 

 tended to make more marked the natural differences in abundance between 

 the different sections. The water from Maumee River has a higher con- 

 centration of nitrogen than that from Portage River, River Raisin, or 

 Detroit River. Being somewhat enclosed, the water is not as rapidly 

 diluted, and the effecx. on the plankton is localized. River Raisin, 

 with lower concentration and more rapid dispersion of its waters than 

 Maumee River, presumably has less local effect on the plankton than 

 that river, but more than Portage River, Without doubt pollution has 

 helped to increase the plankton near Portage River, but that area 

 probably is naturally quite rich. Pollution in Detroit River probably 

 has almost no local effect in increasing the plankton because of the low 

 concentration of nutritive material in the water, and because of the 



199 



