336 



FRESH-WATER ECOLOGY 



water level and wave action: this type of zonation has been ob- 

 served in several continental lakes. In deeper waters the com- 

 munities are more distinct because a zonation develops which is 

 primarily maintained by the light intensity factor. The Limno- 



->EV0LUTI0NARY TrEND- 



C Hard ancient rocks 

 unchanged 



t/3 



« 



o 

 l-l 



CO 



Softer more recent rocks or Soft rocks or much 

 -> some silting >■ silting 



Generally deep 



No O2 decrease with 

 depth at thermocline 



1 

 Poor in dissolved 



minerals 



I 

 Rich in number of 



species 



►Decreasing depth 



►Generally shallow 



O2 decreases with 

 > depth at thermocline 



Minerals in solution 

 increasing 



I 1 



Decreasing number of 



species 



Poor in actual numbers Increasing number of 

 of individuals individuals 



►Rich in dissolved 

 minerals 



I 

 Poor in number of 



«■ species 



I 



Rich in actual numbers 

 >• of individuals 



Desmids abundant -^Diatoms and-^Diatoms and -^ Aster ionella and 

 I desmids Eudorina Cyanophyceae 



I > I I 



)-EuTROPHic waters 



. Oligotrophic waters -^Over many years 



plankton of lakes is not usually of great bulk and is composed 

 principally of various members of the Cyanophyceae, Dino- 

 phyceae, Bacillariophyceae and Chlorophyceae, and according to 

 the nature of the constituents it may exhibit maxima in spring (very 

 commonly), spring and autumn or summer and autumn. 



ASSOCIATIONS OF RIVERS AND STREAMS 



Budde (1928) investigated very thoroughly the mountain streams 

 feeding the Ruhr river. Most of these streams and brooks are trout 

 streams and they can be divided into two regions: 



(i) upper Hildenhrandtia region dominated by H. rivularis, 

 (2) lower Lemanea region dominated by L. fluviatilis. 

 The seasonal facies were studied and were found to be as follows : 

 {a) Spring period characterized by the dominance of diatoms 

 with Ulothrix and Hormidium as subdominants. 



{h) Summer period with Chlorophyceae and desmids pre- 

 dominant. 



