June Distribution 



The total bulk of the samples obtained in June 

 was much less than in the months following, making 

 this the minimum of the plankton supply during the 

 survey. The amounts from the surface net were al- 

 so considerably smaller than those from the bottom 

 net, and a third of them did not exceed 2 cc. At 

 the surface the copepods were considerably more 

 abundant than the cladocerans in the eastern half of 

 the lake, but usually fell much below the latter in 

 the western half (fig. 59). At the bottom there was 

 the same preponderance of copepods in the eastern 

 half of the lake, while in the western half the two 

 were divided almost equally. 



The Cyclops species were distributed quite even- 

 ly throughout the entire lake, but they were a little 

 more abundant in the shallow water along the shores 

 than in the deeper portions of the lake. The percent- 

 ages of those taken in the bottom net were also larg- 

 er than of those taken at the surface. It is worthy 

 of note that the species C. bicuspidatu s was more 

 abundant than C. leuckarti in the eastern half of the 

 lake, but that this proportion was more or less re- 

 versed in the western half. The largest percentages 

 of both species were in the bottom net in the littoral 

 zone between Dunkirk, New York and Erie, Pennsyl- 

 vania. 



During this month the genus Diaptomus outnum- 

 bered the other copepods everywhere except at the 

 bottom in the deeper portions of the lake where it 

 was surpassed by Limnocalanus, and at the surface 

 in the littoral zone where it was surpassed a few 

 times by Epischura . It also was more numerous in 

 the eastern portion of the lake than in the western, 

 and on the American shore than on the Canadian 

 side. Only 2 species of the genus were found during 

 June, and the larger 1 surpasses the smaller in abun- 

 dance as well as in size. One or the other of these 

 2 species was present in every tow taken during this 

 month, and with very few exceptions both species 

 were represented by at least 1 or 2 specimens. The 

 percentages often ran over 70 percent and reached 

 as high as 95 percent, the latter in the bottom net 

 on the American side in the littoral zone at station 

 51. Such an abundance of individuals goes far 

 toward offsetting the relatively inferior size of these 

 2 species and gives the genus considerable economic 

 importance. 



Epischura was more abundant at the surface 

 than at the bottom during June. It was most abun- 

 dant at the surface through the central portion of 

 the lake, while at the bottom its numbers were larg- 

 er at either end of the lake. The percentages in the 

 littoral zone were considerably larger than in the 

 lacustric zone, but there was no appreciable dif- 

 ference between the Canadian and American sides. 

 It was so well distributed that there was but a single 

 station at which it did not appear either in the sur- 

 face or the bottom net, and probably further research 

 would reveal at least a few individuals. 



The genus Limnocal anus was found at the surface 

 in much larger numbers at the eastern end of the lake 

 than through the central and western portions. Its 

 percentages were greatest in samples taken at the 

 bottom in the center of the lake and considerably 

 smaller at either end. There was a decided prepon- 

 derance of this genus on the Canadian rather than on 

 the American side, and it was often found there in 

 water that was comparatively shallow. Being a so- 

 called "relict" form, it would naturally be expected 

 to frequent the cooler water of the deeper portions 

 of the lake. 



The Daphnia species made up very small per- 

 centages of the plankton in the eastern half of the 

 lake, but were present in large numbers in the west- 

 ern half. The contrast between the two portions is 

 so great that it would suggest that the spring breed- 

 ing ground for this genus is in the western end of the 

 lake and probably in Sandusky Bay. Daphnia was 

 found in larger numbers at the surface than at the 

 bottom, especially in the deeper portions of the lake; 

 the 2 species were about equally divided in abundance. 



Holopedium and Sida were virtually absent from 

 the eastern portion of the lake during June, but were 

 present both at the surface and at the bottom in the 

 central and western portions. The latter genus ap- 

 peared at every station, its surface percentages being 

 definitely larger than those at the bottom. The 

 former genus only once obtained a percentage larger 

 than 1 and was frequently absent. 



Leptodora also was more abundant in the western 

 than in the eastern half of the lake, but was seldom 

 present in the meter net tows in sufficient numbers 

 to constitute more than a trace. In the Helgoland 

 trawl samples of 1928, however, it was clearly 



155 



