entire surface samples. The individual specimens 

 of this species were considerably larger than those 

 obtained during June and July, so that from every 

 point of view this cladoceran reached its maximum 



in August. 



Mysis was again confined to the deepest part of 

 the lake below the thermocline and was present in 

 reduced numbers at stations where it was obtained in 

 July. Pontoporeia did not appear in any of the tows 

 made during August, probably due to a restricted 

 distribution rather than to its entire absence. 



September Distribution 



The total bulk of the samples increased 75 per- 

 cent over that of August and was the largest for the 

 4-month period (fig. 62). Only one of the bottom 

 tows fell below 150 cc. , 17 reached 500 cc. or more, 

 and 4 totaled 1, 000, 1,250, 1,500, and 2, 000 cc, 

 respectively. These 4 were in the lacustric zone, 2 

 at the eastern end and 2 in the center of the lake. 

 The large numbers were entirely due to Daph nia, 

 and it comprised two -thirds of the macroplankton at 

 all but a single station. 



Only 2 specimens of Cyclops bicuspidatu s ap- 

 peared in the surface tow at station 31, and 2 other 

 specimens in the bottom tow at station 32, both in 

 the littoral zone near the Canadian shore. Cyclops 

 leuckarti was present at every station either in the 

 surface or in the bottom samples, and often in both, 

 but in such small numbers as to constitute merely a 

 trace. The 2 Diaptomus species appeared at every 

 station and in both the surface and bottom nets, 

 reaching their highest percentages of the plankton in 

 the central and western portions of the lake at the 

 surface. The ratio between surface and bottom sam- 

 ples was about even, the percentages at half of the 

 stations being greater at the surface and in the other 

 half at the bottom. Cyclops minutus did not appear 

 in any of the tows made during this month in the lit- 

 toral and lacustric zones; it was confined to the mar- 

 ginal zone. 



Epischur a showed considerable reduction in its 

 percentages of the total plankton although it was 

 present at every station and usually in both surface 

 and bottom samples. In 26 of the tows the percent- 

 age of this copepod was only 1 and in 13 others it 

 was expressed as a trace. It was more abundant in 



the eastern half of the lake, but there were no dif- 

 ferences between the American and Canadian shores. 



Limnocalan us was no longer confined to the deep- 

 est stations in the eastern portion of the lake, although 

 it was more abundant there than elsewhere. It had 

 spread over practically the entire bottom of the lake 

 except along the shores of the extreme eastern end 

 off Buffalo and the mouth of the Niagara River. It 

 was almost entirely confined to the bottom, being 

 found in the surface tows of only 3 stations, 2 of 

 which were in the deepest part of the lake. Its per- 

 centages of the plankton, however, were in general 

 very small and only 3 times did they reach 2 figures. 



Daphnia constituted the bulk of the macroplank- 

 ton, the percentages of D. pulex in the plankton al- 

 most without exception being larger than those of 

 D. longispina. Indeed in counting the September 

 samples it became evident that the best method was 

 to remove the comparatively few specimens of other 

 Crustacea, keeping tally as they were taken out, and 

 then count the Daphnia specimens without removing 

 them. The distribution was practically uniform, D. 

 pulex showing a little larger percentages in the bot- 

 tom net and J), longispina in the surface tows. 



Sid_a was as widely distributed as in August but 

 its numbers were greatly diminished and its percent- 

 ages of the macroplankton only twice exceeded 5. 

 It was present, however, more often and in larger 

 numbers at the surface than in the bottom samples. 

 Holopedium and Dia phanosoma were not taken in 

 the littoral or lacustric zones during September; the 

 former entirely disappeared, the latter was still 

 present in the marginal zone. Leptodora was almost 

 as widely distributed as in August, but its numbers 

 were reduced. Its percentages nowhere reached 

 above 5 and in 22 of the tows the specimens were so 

 few as to constitute a mere trace. 



Mysis^ and Pontop oreia were both present in the 

 deeper portions of the lake, so that the absence of 

 the latter genus from the August samples must be due 

 to very restricted distribution. The bottom nets at 

 the deepest stations in the lake happened to hit the 

 Pontoporeia habitat in June, July, and September, 

 but missed it in August. 



161 



