Seasonal Range 303 



of summer brings on the remaining algae, first the greens 

 and then the blue-greens, in regular seasonal succession. 

 It brings with them a wave of the flagellate Ceratium, 

 which, being much less eaten by animals than they, 

 often gains a great ascendency, just as the browsing of 

 grass in a pasture favors the growth of the weeds that 

 are left untouched. Green algae reach their maximum 

 development in early summer, and blue-greens, in mid 

 or late summer, when the weather is hottest. 



With the cooling of the waters in autumn, reproduc- 

 tion of summer forms ceases and their numbers decline. 

 The fall overturning and mixing of the waters usually 

 brings on another wave of diatom production, followed 

 by the long and gradual winter decline. This is often 

 accompanied, as in the spring, by abundance of Dino- 

 bryon. The flagellate Synura (see fig. 30 on p. 103) is 

 rather unusual in that its maximum development occurs 

 often in winter under the ice. 



The coming and going of the plancton organisms 

 has been compared to the succession of flowers on a 

 woodland slope; but the comparison is not a good one; 

 for these wild flowers hold their places by continuously 

 occupying them to the exclusion of newcomers. The 

 planctonts come and go. They are rather to be 

 likened to the succession of crops of annual weeds in a 

 tilled field; crops that have to re-establish themselves 

 every season. They may seed down the soil ere they 

 quit it, but they may not re-occupy it without a strug- 

 gle. And as the weeds constitute an unstable and 

 shifting population, subject to many fluctuations, so 

 also do the plancton organisms. They come and go; 

 and while on their going we know that when they come 

 again, another season, they will probably present col- 

 lectively a like aspect, yet the species will be in different 

 proportions. 



