SYNTHETIC CHARACTERS OF THE COMMUNITY 75 



Analysis of Floating Communities and of the Phytoedaphon. — 

 With plankton, as with higher plants, communities may be character- 

 ized by density, dominance, sociability, vitality, layering, periodicity, 

 constancy, and fidelity. The definitions given for communities of higher 

 plants may be carried over to the plankton communities (c/. Allorge, 

 1922; Denis, 1925, pp. 68-73; Steiner, 1925; Huber-Pestalozzi, 1925). 



PJiyto-plankton. — Density and dominance of plankton can be only 

 approximately given by net catches and may be expressed in words or 

 figures (1 = very rare; 5 = very abundant, dominant). More exact 

 results are obtained by collecting samples in flasks, so that the plankton 

 is not concentrated by filtration. Immediately after taking, 50 to 

 100 cc. of the fluid is fixed with formalin. Before examining, the 

 sample is carefullj^ stirred up, and 1 cc. at a time is counted in a 



10m. 



-Plankton stratification in Lake Zurich near Rapperswii, Oct. 17, 1924. 

 Number of individuals or of colonies per liter of lake water. 



Kolkwitz plankton chamber. The number of organisms and species 

 present is reckoned per liter of water. Obviously this method is 

 useful only where the density of individuals is rather high. 



With plankton organisms the aggregation of individuals into groups 

 and colonies is not unusual and must be considered {Dirwbryon 

 bunches, Oscillatoria threads, Tabellaria colonies, floating gelatinous 

 masses) . Messikommer has made use of a scale of 6 deg. to express the 

 amount of sociability of plankton organisms. 



Denis (1925, p. 72) considers it also very important to note the 

 vitality and stage of development of the species (vegetative condition, 

 zoospores, aplanospores, etc.). According to him, vitality and abun- 

 dance of algae are most intimately connected. Layering also plays a 

 special role in plankton. To show this, a block diagram is found useful 

 (Fig. 42). 



Since the recognition of the importance of plankton for aquatic 

 life in the sea and larger inland waters, the study of its periodicit}^ 

 has been vigorously pursued. It is expressed in the same manner as 

 layering. Graphs may express the annual and seasonal variations 

 (Fig. 43). The question remains open as to how far these annual 

 variations are true aspect phenomena and how far they represent the 

 replacement of one community by another. 



