Distribution in Depth 307 



Shoreward Range — Few plancton organisms are 

 strictly limited to life in open water. Most of them 

 occur also among the shore vegetation in ponds and 

 bays and shoals. They are very small and swim but 

 feebly, and there is room enough for their activities in 

 any pool. They mostly belong in the warm upper 

 strata of the lake, and similar conditions of environ- 

 ment prevail in any pond. It is the deep waters of the 

 lake that maintain uniform conditions of low and 

 stable temperature, and scanty light; and it is the 

 organisms of the deeper strata that do not appear in the 

 shoals. 



Hence, though the aquatic seed-plants pushing out 

 on a lake shore are stopped suddenly at given depth, 

 as with an iron barrier, the more simple and primitive 

 algae of the plancton range freely into all sorts of suit- 

 able shoreward haunts. We shall meet with them 

 there, commingled with numberless other forms that 

 have not mastered the conditions of the open water. 

 In each kind of situation (pond, river or marsh has each 

 its plancton) we shall find a different assemblage of 

 species. In all of them we shall find the planctonts are 

 less transparent; in none of them will there be quite 

 such uniformity, from place to place, as is found in the 

 population of the open waters of the lake. 



Distribution in Depth. Since plancton organisms 

 tend to be uniformly distributed in a horizontal plane 

 one may ply his nets at any point on a lake with the 

 expectation of obtaining a fair sample ; but not so with 

 depth, except at times when the waters are in complete 

 circulation. A net drawn at the surface would make 

 a ver}" different catch from one drawn at a depth of 

 fifty feet. Certain species found in abundance in the 

 one would not be represented in the other. The 

 organisms of the lakes tend to be horizontally stratified. 



