It was not possible to determine the depth at which 

 production can take place in Lake Erie. The presence 

 of large numbers of diatoms in the deeper levels at the 

 end of a productive period is not necessarily an indica- 

 tion that production is taking place there. The sinking 

 to the bottom after active production has ceased may 

 be slow and the vertical gradient at such times would 

 not yield a true picture of the normal vertical range. 

 Again, the vertical range at the beginning of the pro- 

 ductive period may not be indicative of conditions at 

 the peak. It is evident, however, that except for a 

 very small sector, diatom production takes place from 

 the surface to the bottom in Lake Erie. 



Green algae were found most abundant in the upper 

 10 meters, but in shallow areas the vertical gradient 

 was hardly perceptible. A seasonal change in the ver- 

 tical range was observed in the blue -green algae. They 

 were concentrated in the upper levels in June. Little 

 difference was found between the surface and bottom 

 in the eastern area in August. The rotifers showed lit- 

 tle vertical variation except a tendency to concentrate 

 in the upper levels. 



Factors of Production 



The time of appearance of major plant pulses in 

 any area in the lake is largely a function of light and 

 temperature; the degree of response is largely a func- 

 tion of available food stuffs. Thus the extreme 

 fluctuations found on occasion, usually in limited 

 alongshore areas, may be considered indicative of a 

 variable food supply, but the general distribution 

 over the lake as a whole can most closely be cor- 

 related with physical environment. 



Since the present survey was designed to deter- 

 mine the pelagic flora of the lake and its quantita- 

 tive and qualitative distribution during the period from 

 June to September, no attempt has been made to de- 

 termine the maximum plant population which Lake 

 Erie is capable of supporting. In the limited time 

 available and with so large an area under considera- 

 tion, such a program could not be included in the 

 scope of the present investigations. However, the 

 rapidity with which the waters of the lake are depleted 

 of carbon dioxide and the quantitative figures on the 

 plants themselves, considering the rich fauna of preda- 

 tory forms, indicate no artificial Limiting factors 

 introduced by man. 



MACROPLANKTON 



Special consideration was accorded the macro - 

 plankton in the Lake Erie investigations, first because 

 of its importance as food for fishes, and secondly be- 

 cause the presence of artificial environmental con- 

 ditions unfavorable to animal life can be more readily 

 detected in plankton than in fishes. 



The macroplankton of the Great Lakes plays a much 

 more important role as a direct source of food for adult 

 fishes than that of the sea. This is clearly indicated by 

 the relatively large number of lake species which depeni 

 almost solely upon this source of supply. Such a con- 

 dition is necessitated by very fundamental differences 

 between the invertebrate communities of the two en- 

 vironments. Owing to the absence of an intertidal zone 

 there is not the rich benthic community of larger in- 

 vertebrates along the margin of the shore of Lake Erie 

 that one finds in the ocean. In the neritic zone, also, 

 although small forms such as insect larvae are particu- 

 larly abundant at times, the vast numbers of larger 

 benthic species which support practically aU adult non - 

 migratory ocean fishes are conspicuously scarce at least 

 in Lake Erie. The scanty fauna of crayfish, shrimp, and 

 molluscs, appearing in the bottom trawls, may be part- 

 ly accounted for by the concentration of predatory fishes, 

 but the fact remains that to maintain themselves the 

 vertebrates must depend on another source of supply, 

 and this is provided in the macroplankton. 



Macroplankton Production 



In considering the production of pelagic Crustacea 

 in Lake Erie, the seasonal fluctuation in the adult popu- 

 lation serves as the basis for our calculations. Data 

 were not obtained on the time of appearance of the 

 larval stages, but development is rapid and actual pro - 

 duction can be considered to have taken place several 

 weeks earlier than the present records indicate. 



The summer repopulation of the lake begins in the 

 southwestern portion with the first rise in temperature. 

 Here the response to meteorological conditions Is most 

 rapid. An eastward trend follows, the areas along shore 

 being influenced first. Gradually the deeper central 

 portions of the lake are affected. The last portion to 

 be affected is the northeastern region comprising the 

 Deep Hole and the Canadian littoral. This apparent 

 easterly migration of certain species as the season 

 progresses is probably due more to a differential response 



178 



