Although the periods of time represented in Tables 8l and 82 

 are not exactly the same, there seems to be no serious objection to 

 combining the means to obtain a two-year mean for each section. These 

 means for nearly the same periods of the two years are (omitting 

 decimals) : Maumee Bay Section, 66 per liter; River Raisin Section, Ii6 

 per liter; Detroit River Section, h per liter; Island Section, 38 

 per liter. Now, if the mean a bundance in the Detroit River Section 

 be regarded as unity, the relative abundance in the other sections 

 would be as follows: Maumee Bay, l6; River Raisin, 12; Island, 10, 

 In other words there were l6 times as many Crustacea in the Maumee 

 Bay Section as in the Detroit River Section; in the River Raisin and 

 Island Sections there were, respectively, 12 and 10 times as many as 

 in the Detroit River Section. 



In preparing Tables 8l and 82, data for Station l59, in the 

 Portage River Section, were omitted because one or moi'e of thetwo-wgek 

 periods in each year were not represented at that station. The mean 

 count in 1929 for four two-week periods which lie within the total 

 period covered in Table 81 was J4O per liter (not including Bosmina) . 

 This figure agrees closely with those for the River Raisin and 

 Island Sections in Table 8I, In 1930 for five periods (early and late 

 July, early August, early and late September) the mean count was 

 57 per liter. For the same periods the mean counts in the other 

 sections were: Maumee Bay, 6O; River Raisin, 1(7; Detroit River, 2; 

 Island, 39. The probable significance of these comparative figures 

 will be commented upon later. 



The most striking fact brought out in Tables 8I and 82 is 

 that the Crustacea are very rare near the mouth of Detroit River, as 

 compared with other parts of the lake. There may be a number of 

 factors responsible for this marked inequality in horizontal dis- 

 tribution, but in all probability the factor of greatest importance 

 is that of food. It is more than likely that the scarcity of 

 Crustacea in lower Detroit River is the direct result of a similar con- 

 dition in Lake St. Clair. This condition in Lake St. Clair is believed 

 to be the result of the small amoimt of food available to the Crustacea. 

 Samples taken on September 23, 1930 show that the phytoplankton, upon 

 which the Crustacea feed, was scanty as compared with the Island 

 Section at about the same time. Poverty of phytoplankton in Lake 

 St. Clair is indicated further by the studies of Reighard (I893 and 

 l89li) , and by the fact that Station 126, near the mouth of Detroit 

 River, almost invariably yielded a small amount of phyt,oplankton, 



2U7 



