on the American and Canadian sides, particularly in 

 the area east of Long Point. 



Lacustric zone. --This zone is characterized by the 

 "arctic relict" forms Mysis and Pontoporeia which are 

 found only in the deep cold water after June, and by 

 Limnocalanus which becomes more and more restricted 

 to deeper areas as the summer advances. Differences 

 in vertical distribution due to the thermocUne are much 

 more pronounced in the lacustric zone. These differ- 

 ences are both qualitative and quantitative. 



Macroplankton Community 



The macroplankton community of the open lake 

 consists of 8 copepods, 7 cladocerans, 1 mysldacean, 

 and a few insect larvae. Only 11 species can be con- 

 sidered of major significance in the economy of the 

 lake in summer. These in order of their abundance 

 over the lake as a whole are: Daphnia pulex, D. longi - 

 spina, Diaptomus ashlandi, D. sicilis , Limnocalanus 

 macrurus, Epischura lacustris , Cyclops bicuspidatu s, 



C. leuckarti, Leptodora kindtii , Mysis relicta , and 

 Sida crystallina. 



Daphnia pulex was the most important macro- 

 plankton organism because of its size and abundance 

 during the summer months, when together with D. 

 longispina it formed considerably more than half of the 

 plankton of the littoral and lacustric zones. The nor- 

 mal trend of plankton production in the lake was well 

 illustrated in the seasonal distribution of these 2 species 

 in 1929. D. longispina became abundant first in the 

 southwestern portion of the Great Plains Basin with the 

 rise in temperature in June. (At this time the copepods 

 Diaptomus sicilis and J), ashlandi had passed their peak 

 in the western area but were still increasing in the 

 eastern part of the lake. ) Gradually it spread out over 

 the Central Basin and along the American shore into 

 the eastern area. In June smaU numbers were found 

 all along the south shore in the Deep Hole region but 

 were entirely absent in the middle of the lake and on 

 the Canadian side. By July it had extended its range 

 to cover the whole lake, although in the northeastern 

 sector (Canadian side) it was still outnumbered by 

 Diaptomus ashlandi . 



Daphnia pulex increased rapidly in the Central 

 Basin during July and quickly expanded eastward. Since 



D. longispina had by now passed its peak, D. pulex 

 soon became the dominant member of the lake com - 



munity everywhere except below the thermocline in 

 the Deep Hole where it was outnumbered by Limno- 

 calanus. By September it had begun to decline in 

 the southwestern portion of the Central Basin but in the 

 east the numbers were still increasing at the termina- 

 tion of the investigations. In 1928, due probably to 

 higher temperatures throughout the summer, D. pulex 

 terminated its maximum earlier and in September was 

 exceeded b y Diaptomus ashlandi. 



Of the copepods, Diaptomus ashlandi and D. 

 sicilis form important and sometimes dominant mem- 

 bers of the community for a limited period following 

 their breeding seasons. D. sicilis became the domi- 

 nant species for a short time in June and then declined 

 slowly during the summer, p. ashlandi increased dur- 

 ing the summer and in September 1928 dominated the 

 catch. In 1929 it had not exceeded Daphnia at the 

 termination of the investigations. 



Although their distribution is greatly restricted 

 during the warm season, Limnocalanus macrurus and 

 Mysis relicta are important because of their size and 

 abundance. Limnocalanus is generally distributed at 

 aU levels in Jime, but first seeks the bottom and then 

 gradually becomes restricted to the eastern area as 

 the bottom temperature in the Central Basin rises above 

 14° C. Mysis responds more readily to rising tempera- 

 ture than Limnocalanus and by June 1929 had retreated 

 to the Deep Hole where it remained during the rest of 

 the summer. Together these 2 species form the bulk 

 of the macroplankton below the thermocline. 



Epischura lacustris reached Its maximum in the 

 littoral zone during the period of rising temperatures 

 in June and declined during July, August, and Septem- 

 ber. Although a warm water species (Marsh 1903) it 

 did not favor the highest summer temperatures in Lake 

 Erie and as the season advanced, it penetrated deeper 

 and became more and more restricted to the cool east- 

 ern area. 



Cyclops bicuspidatus and C. leuckarti were never 

 the dominant species in the littoral or lacustric zones 

 although each formed at times a significant part of the 

 community. C, Bicuspidatus reached its maximum 

 early and in 1929 was decreasing in the southwestern 

 area by June 19. It had almost entirely disappeared 

 by August. C. leuckarti increased over a wide area 

 during the summer, reaching its maximum in August 

 and declining to a mere trace in September. 



188 



