Leptodora kindtil and Sida crystallina reached 

 their maxima in midsummer and both had their cen- 

 ter of production west of Long Point, [yeptodora was 

 present in small numbers throughout the lake until 

 August when it increased rapidly and formed up to 

 33 percent of the hauls in the Great Plains area. The 

 maximum was short lived and in September it no- 

 where exceeded 5 percent of the catch. Most of the 

 collections yielded only occasional individuals. Sida 

 reached its peak in July when it comprised up to 94 

 percent of the community in some parts of the Cen- 

 tral Basin. Here it remained abundant throughout 

 August and declined during September. Like Lepto - 

 dora it occurred at all times in small numbers in the 

 eastern area. 



Vertical Distribution 



The extent of the area investigated prevented 

 special observations on vertical distribution and diur- 

 nal migration as all biological collections were made 

 during dayKght. These results, however, indicate 

 that light forms but one of several factors governing 

 vertical distribution in Lake Erie. Although the larg- 

 est volumes were generally obtained at the bottom, 

 uniform vertical diffusion and concentration at the 

 surface were not uncommon even on the brightest 

 days in midsummer. To a large extent the latter 

 condition appears to be closely correlated with tem- 

 perature. Warm -water species such as Epischura show 

 a tendency to remain near the surface in offshore 

 waters until the lower levels become heated. They 

 then gradually increase their vertical range as the 

 season progresses. 



In contrast with Epischura , Limnocalanus, which 

 during the early summer was often found at the sur- 

 face during the day, later became restricted to the 

 hypolimnion and although verification will be neces- 

 sary, it is possible that a large part of the population 

 does not rise to the surface at night. This suggestion 

 is based on observations on ocean forms made in 

 coastal waters south of Cape Cod where the domi- 

 nant members of the spring plankton community of 

 the neritic zone are found in the lower levels off- 

 shore in midsummer. A limited diurnal migration 

 may take place during this season within their tem- 

 perate range but repeated night haiils failed to re- 

 veal any at the surface. Since certain freshwater 

 residents favoring low temperatures, such as Mysis 

 relict a, are known to rise to the surface at night 



even in midsummer in some lakes (Birge and Juday 

 1927), it is questionable if very general application 

 can be made from the findings in any single body of 

 water. Results of previous observers indicate great 

 variation in the diurnal range of the same species in 

 different lakes. 



It appears probable that the factor of feeding will 

 also have to be considered in interpreting the verti- 

 cal distribution of macroplankton in Lake Erie. This 

 factor has recently been commented on by Worthing - 

 ton (1931) who observed food relationships in the ver- 

 tical distribution of pelagic Crustacea in Lake Vic- 

 toria Nyanza and Lake Lucerne. In August 1929 when 

 the lowest oxygen values in Lake Erie were recorded 

 from the bottom in the black mud area off Rondeau, 

 the 4 stations (41 to 44) in contrast with the rest of 

 Lake Erie yielded almost nothing at the surface. At 

 each of these stations 1 cc. of plankton was taken in 

 5 -minute surface hauls with a meter net. The fact 

 that the entire community at this point was concen- 

 trated at or near the bottom while elsewhere, for the 

 most part, it was more evenly diffused vertically, 

 would suggest that the cladocerans and the copepods 

 were feeding on organic matter in or on the black mud. 



Vertical variation frequently occurred in the same 

 community under conditions difficult to explain in 

 terms of physical environment. Such conditions are 

 indicated in table 23, showing the vertical distribution 

 of Diaptomus ashlandi on 2 consecutive days in Aug- 

 ust 1928. The weather on both days was bright and 

 clear with a moderate wind, but on August 28 D. 

 ashlandi was found concentrated at the surface while 

 on August 29, at 5 of the 6 stations, it was concen- 

 trated near the bottom. On August 29 bottom tem- 

 peratures were lower at stations 05 to 07A than at sta- 

 tions 01 to 04 but were directly comparable at sta- 

 tions 09 to 11. 



"Marine reUct" Community 



The "marine relict" species of the cold-water 

 community in Lake Erie include Rhizosolenia eriensis , 

 Stephanodiscus niagarae, Limnocalanus macrurus, 

 Pontoporeia affinis , Mysis relict a, and Triglopsis 

 thompsonl. As in the case of European "marine 

 relicts" it is evident that the transfer from the ocean 

 to their present habitat in Lake Erie did not neces- 

 sarily take place in the same manner or at the same 

 period of time. The community consists of two major 



189 



