stations and consistently rare at others. Comparisons of seasonal distribu- 

 tion of individual genera in 1929 and 1930 indicate that the lack of uniform- 

 ity was not such as to invalidate a determination of average abundance in the 

 section based on samples from several stations. 



Vertical distribution was studied only during the hours of daylight, so 

 that nothing is known regarding diurnal migrations. In the daytime the adult 

 Crustacea were usually rare at the surface and near the bottom, and were most 

 abundant at some intermediate depth. Nauplii and rotifers appeared not to 

 avoid the water near the surface, but were commonly concentrated at more than 

 one level. There were numerous exceptions to any general rule regarding ver- 

 tical distribution of the zooplankton organisms. 



Only in the Island Section was sampling continued over a sufficiently 

 long period to show seasonal distribution clearly. Nothing is known defi- 

 nitely regarding abundance in the months of December, January, February, and 

 March, but there are reasons for believing that the Crustacea are rare during 

 that period. During the remaining months the adult Crustacea were rare in 

 spring and autumn, and were most abundant in surmer. In 1930 copepod nauplii 

 were most abundant in late spring, and this probably was true in 1929 also. 



The four most prominent general of Crustacea were Cyclops, Diaptoaus, 

 Daphnia, and Diaphanospma. For the period late May - early October for the 

 years 1929 and 1930, the mean counts per liter in the Island Section were as 

 follows: Cyclops, lOj Diaptomus, 6; Daphnia, U: Diaphanosoma, 1. The cor- 

 responding mean for the nauplii was l6 per liter. Comparisons of these 

 figures with corresponding figures from a typical eutrophic lake and a typi- 

 cal oligotrophic lake show that the Island Section holds an intermediate posi- 

 tion with respect to abundance of Crustacea. Since eutrophic lakes are char- 

 acteristically rich in plankton and oligotrophic lakes are poor. Western Lake 

 Erie in the Island Section may be described as "moderately rich" in plankton 

 Crustacea. 



Large and highly consistent inequalities in horizontal distribution exist 

 in Western Lake Erie as a whole. For the months of July, August, and Septem- 

 ber of 1930, the mean number of Crustacea in the Detroit River Section was 

 l/l3 of that in the Island Section; l/l7 of that in the River Raisin Section; 

 and 1/20 of that in the Maumee Bay Section. Differences of similar magnitude 

 were found for about the same period of time in 1929. These differences in 

 abundance of the plankton Crustacea are believed to be dependent upon the 

 amount of food available to them, for in 1930, and probably in 1929 also, the 

 different sections just mentioned held the same positions with respect to 

 abundance of phytoplankton as they did with respect to plankton Crustacea. 

 That is, the Maumee Bay Section was first in abundance of both kinds of 

 plankton organisms, the River Raisin was second, the Island Section third and 

 the Detroit River Section fourth. The Portage River Section is not included 

 in the list because it is represented by less adequate data. 



