high carbon dioxide occurring in the open lake dur- 

 ing the summer of 1929 were not limiting factors to 

 pelagic animal production or distribution. The com- 

 munity at stations 41 and 44 in the Rondeau region 

 was found concentrated at the bottom and exceeded 

 in volume the average for the Central Basin. 



Methyl -orange alkalinity was uniform through- 

 out the lake, the surface values being consistently 

 lower. There was a very slight decrease during the 

 summers of both years, probably due to the precipi- 

 tation of calcium carbonate caused by the removal 

 of carbon dioxide. Some of the precipitate is prob- 

 ably recovered when the carbon dioxide is again 

 introduced in the lower levels by vertical mixing. 

 This removal of the carbon dioxide resulted also in 

 a gradual increase in pH, the bottom averaging some- 

 what lower than the surface. 



Turbidity tests indicate relatively clear water 

 for the lake as a whole. The highest figures were 

 from the vicinity of the Deep Hole, as expected, 

 for here the bottom is covered with fine clay silt 

 and is subjected to great animal activity and active 

 movement of the bottom water. The surface waters 

 cleared rapidly over the whole lake and the bottom 

 waters gradually during the summer. 



The seasonal trend of the lake from June to 

 September shows an increase in oxygen saturation, 

 albuminoid nitrogen, free ammonia, and nitrates, 

 and a decrease in dissolved carbon dioxide, pH, 

 methyl orange alkalinity, and turbidity. 



The bottom levels in the Deep Hole contrasted 

 with the lake as a whole are characterized by rela- 

 tively low oxygen, free carbon dioxide at all times, 

 low pH, and high turbidity. 



MICROPLANKTON 



Horizontal Distribution and Seasonal 

 Variation 



An outstanding characteristic of the summer 

 plankton of Lake Erie is the horizontal variation in 

 both plants and animals. Four major groups of micro - 

 plankton, diatoms, green algae, blue -green algae, 

 and rotifers, occur in abundance at certain seasons of 

 the year, but never do they appear to be uniformly 

 distributed over the entire lake and in most instances 

 their maxima do not occur at the same time. 



Horizontal distribution at any time in Lake Erie 

 must be related to seasonal variation, for the latter 

 does not occur uniformly throughout the lake. There 

 appears to be a correlation between the seasonal 

 changes in temperature and microplankton response. 

 As the shallower areas of the lake respond first to 

 rising or falling temperatures, the plant pulses become 

 noticeable first in the western area and gradually ex- 

 tend eastward. 



When the investigation began in 1929, the vernal 

 diatom maximum was drawing to a close in the west- 

 ern pcation of the Great Plains Basin although diatoms 

 were still fafrly abundant over a considerable portion 

 of the lake. The eastward trend became evident by 

 July, with the diatom production concentrated in the 

 region east of Long Point. The autumnal maximum 

 made its first appearance in the western area early in 

 September of 1929 and by the 19th had spread as far 

 east as Fairport, being most pronounced in the very 

 shallow alongshore zone. Unfortunately, the termin- 

 ation of the observations at this time did not permit a 

 determination of the date of the response at the east- 

 ern end. 



As reported by previous observers (Birge and Juday 

 1922) the diatoms were found to respond most vigor- 

 ously to favorable environmental conditions and are 

 the dominant forms in the vernal and autumnal maxima. 

 The green algae paralleled in a general way the fluc- 

 tuations of the diatoms but the blue -green algae ap- 

 peared to vary inversely with them. In June in the 

 Central Basin blue -green algae were fairly abundant 

 but entirely absent in the eastern area. In August they 

 appeared as the dominant midsummer group at the 

 time that the diatom production was at its minimum. 

 Whether this indicates a biological correlation between 

 the two groups or a differential response is not known, 

 although the latter appears more probable. 



Vertical Distribution 



Uniform vertical distribution was found in the 

 shallow areas which comprise the greater part of the 

 lake. Only in the vicinity of the Deep Hole did the 

 diatoms, whose range exceeds that of the other groups, 

 decrease in numbers. A marked decline was observed 

 below 50 meters. The average number of diatoms per 

 liter in 1929 was 4, 000 at the surface and 4, 300 at the 

 bottom. (The vertical gradient in the limited deep 

 area is in this instance obscured. ) 



177 



