Blue-greens . The seasonal distribution of the blue-green 

 algae in 1929 and 1930 is shown graphically in Fig. 17. On page 172 

 the curves are compared mathematically. They agree very closely for 

 comparable periods of time, even more closely than the diatoms or 

 greens. 



The only marked difference between the two years was in the 

 earlier appearance of large numbers in 1930. This point was noted for 

 the greens also, and probably the same explanation applies in both 

 cases, namely, the higher water temperatures in 1930. Another feature 

 in common with the greens is the greater abundance of blue-greens in 

 May as conpared with April aid June of 1930. There is no indication of 

 this in the curve for 1929. Earlier sampling in 1929 might have de- 

 tected the phenomenon, but that it was present seems doubtful from the 

 fact that the water temperatures in May, 1929, were lower than in May, 

 1930, and we should expect the increase to come later rather than 

 earlier. On the whole it appears probable that there was no vernal 

 increase of blue-greens or greens in 1929* 



In the majority of lakes, blue-greens reach their peak in 

 late summer or in early aut\mn, as in Western Lake Erie, while greens 

 culminate somewhat earlier. Temperature is generally regarded as the 

 most important factor in determining the time of greatest abundance 

 of both of these groups. Apparently, then, the greens prefer a 

 slightly higher temperature than the blue-greens (but see G. M. Smith, 

 192h, p. 110). The data from Western Lake Erie give no evidence of 

 this preference, for in both years the blue-greens agreed very closely 

 with the greens with respect to the time of greatest abundance. This 

 remarkably close agreement indicates that the two groups react in the 

 same way to the controlling factor or factors in the environment in 

 this lake. The reason for this agreement in Western Lake Erie, in 

 contrast to the usual situation, is not known. 



The composition of the group was somewhat similar in the two 

 years. In 1929, Coelospherium, Oscillatoria, Merismopedia, and 

 Gomphosphaeria made important contributions to the plankton, Merismopedia 

 being the most conspicuous form. In 1930, Gomphosphaeria was very 

 rare, but the other three genera were abundant again. The principal 

 difference in the distribution of these three was that in 1930 they 

 maintained their abundance for a longer time than in 1929. The rarity 

 of Gomphosphaeria in 1930 was compensated for by the great abundance 

 of Microcystis, The latter was the most abundant blue-green during 

 late September and early October, 



169 



