486 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



in general is the movement of the water — particularly the mixing 

 action of eddy-diffusion currents. Non-motile forms are kept sus- 

 pended primarily by these, and under conditions of active eddy- 

 diffusion are distributed more or less throughout the zone of turbu- 

 lence — apart from a tendency to decrease just below the surface, 



MAY 22 

 R N C A 



500 50 10 1100 



JULY 20 

 N C A 



10 8 3400 



R 



340 



OCT. II 



D Oo T 



25800 



4-9 14-4 



Fig. 165. — Diagrammatic representation of the plankton in a Wisconsin lake during 

 May to October, the numbers being those of organisms per litre. R = Rotifers ; 

 N = Nauplii ; C = Crustacea ; M = depth in metres ; A = Algae other than 

 Diatoms ; D = Diatoms ; O2 = dissolved oxygen in cubic centimetres per litre; 

 T = temperature in degrees Centigrade ; tr. = trace. (After Welch.) 



where the latter acts as a brake on eddy-diffusion currents and hence 

 allows depletion by sinking. There may also be marked differences 

 on windy and still days, as indicated in Fig. 166 of the representation 

 of a Blue-green Alga of low specific gravity, which accumulates at 



