DISTRIBUTION AND SEASONAL VARIATION 

 OF MAJOR MICROPLANKTON GROUPS 



Analyses of the samples obtained on the various 

 cruises show considerable variation in both the quality 

 and quantity of microplankton in different parts of the 

 lake and at different times. The data shown on the 

 maps and graphs (figs. 44-52) are portrayed in terms 

 of the aggregate number of individuals or colonies per 

 liter of all species belonging to the several groups 

 found at each station. The groups emphasized are the 

 diatoms ( Bacillarial es), the green algae ( Isokontae , 

 Heterokontae , Chrysophyceae, Dinophyceae , and 

 Euglenineae ), the blue -green algae ( Myxophyceae ), 

 and the rotifers ( Rotifer a) . 



The method of plotting the data is as follows: 

 The number of organisms per liter is taken as the vol- 

 ume of a sphere whose size is represented as a circle. 

 It should therefore be remembered that the size dif- 

 ferences of these circles represent volumes of plank- 

 ton which vary with the cube of the radius. In the 

 supplementary graphs (figs. 46, 51, 52) the cube root 

 of the number of organisms per liter have been plot - 

 ted for comparison of the abundance of the more im - 

 portant groups. 



Vertical Distribution 



In general the data indicate uniform vertical 

 distribution of the phytoplankton in the shallow areas. 

 Only in the deep area off Long Point did the plank- 

 ton decrease in the lower levels. In 1928 the dia- 

 toms were found to extend into somewhat greater 

 depths than the other groups of phytoplankton (Burk- 

 holder 1929). Even the diatoms decreased markedly 

 at 50 meters where temperature and light were less 

 favorable for the vital processes. There was a de- 

 cided decrease in the green algae below 10 meters 

 in these deep waters, but in the shallow areas very 

 little vertical difference was found in their quantita- 

 tive distribution. The average number of diatoms for 

 all stations observed in the summer of 1929 amounted 

 to 4, 000 per liter at the surface and 4, 300 per liter at 

 the bottom. Since the greater part of Lake Erie is rel- 

 atively shallow, these averages, of course, obscure 

 the local conditions in deep water where there is a 

 paucity of green forms at the bottom. 



During June when the blue-green algae were ex- 

 tremely abundant in the central western area (fig, 48), 



almost the entire volume was concentrated at the 

 surface. This is due to the relatively low density of 

 the elaborated cellular contents, which resulted in 

 the "bloom" remaining suspended in the upper layer. 

 Later in the season, e. g. in the August pulse of the 

 eastern area, there was little average difference in 

 the abundance of blue -green algae at the surface and 

 bottom. 



The rotifers were usually found in greater num- 

 bers at the surface, if there was any vertical gradient 

 at all. The majority of stations, however, showed 

 remarkable uniformity in the vertical distribution of 

 this group. Notable exceptions may be pointed out in 

 the case of the samples collected in Long Point Bay, 

 Port Maitland, and south from Port Colborne. At 

 these stations Anapu s ovalis was very abundant and ap- 

 parently thrived better at the surface. 



Horizontal Distribution 





Environmental conditions influencing the growth 

 and reproduction of the plankton vary widely from 

 place to place throughout Lake Erie. Since the time 

 required in running each monthly cruise from Buffalo 

 to Put -in-Bay, Ohio, was about 2 weeks, it should be 

 remembered that the factors of seasonal periodicity, 

 as well as simultaneous horizontal differences in the 

 physical and chemical constituents, were probably 

 active in causing the variations plotted on the maps. 



Diatoms were relatively more abundant in the 

 shallow waters near shore in June (fig. 44), decreasing 

 at the deeper stations and also at certain shallow sta- 

 tions, e.^. off Rondeau, Ontario, and Lorain, Ohio. 

 In July the greatest numbers occurred in the eastern 

 part of the lake, mere traces being found at the west- 

 ern stations along the American shore. 



The horizontal distribution of diatoms nearly 

 approached uniformity in August (fig. 45). Only off 

 F airport, Ohio, were large numbers taken in the 

 catches, the rest of the stations showing a remarkable 

 diatom paucity at this time. 



Relatively few diatoms were taken at the east- 

 ern stations in early September, but during the second 

 week diatoms were very abundant in the western area. 

 The single day's run from Port Stanley, Ontario, to 

 Ashtabula, Ohio, shows a remarkable horizontal gradi- 

 ent, the diatoms becoming scarcer at the stations in 

 Ohio waters. 



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