COMMUNITIES IN STANDING WATERS 



339 



the influence which the thermocline has on the distribution of the 

 plankton, and the secondary concentration of Crustacea over the bottom 

 layer which is poor in oxygen. The majority of rotifers occur in a very 

 definite order of strata in the deep waters (Norway) : Conochilus in 

 to 0.25 m.; Notholca longispina and Polyarthra platyptera in the 

 uppermost 5 m.; Anuraea cochlearis seldom above 5 m., mostly below 

 10 m.; Ploesoma hudsoni rarely in the uppermost 5 m., occasionally 



Number 



100 150 



200 



17.90° 



Fig. 100/ — Quantitative distribution of Crustacea in Lake Sakrow near Pots- 

 dam, in number of individuals per liter. Depth in meters at left, corresponding 

 temperatures at right; T ., thermocline. After Behrens. 



even below 10 m., in one case as low as 25 m. ; only a few individuals 

 of Anuraea aculeata are found above 10 m., the majority live at about 

 25 m. 36 The nannoplankton is similarly stratified. Such stratification is 

 regularly changed by night migration when many forms move toward 

 the surface, and is re-formed at daybreak. 



A seasonal variation in the appearance of the plankton is also 

 brought about by the change in body form to which various plankton 

 animals, especially Cladocera and the rotifers, as well as the flagellate 

 Ceratium hirundinella, are subject because of the influence of the 

 amount of nutriment and indirectly because of the temperature and 

 light. Such cyclomorphoses naturally appear in a most pronounced 

 manner where great variations of temperature, and consequently 



