FRESH WATER ECOLOGY; SOIL ALGAE 359 



(i) Attached communities (frequendy termed the Benthos) : 

 A B 



Permanently attached Temporarily attached 



{a) On Uving material (Epiphytic or epizooic). 



(h) On dead organic material. 



(c) On inorganic material (Epilithic). 



{d) In the silt (EpipeUc). 



(2) Floating macro-communities (Pleuston) : 

 {a) Originating from loose bottom forms. 

 (h) Originating from epiphytic forms, 

 (c) Wholly floating throughout. 



(3) Loose-lying communities of the bottom. 



(4) Plankton or floating communities : 

 {a) Limnoplankton of lakes. 



(h) Potamoplankton of slow rivers, 

 (c) Cryoplankton of the eternal snows. 



In this last category we have red snow due to the presence of 

 Chlamydomonas nivalis', yellow snow with a flora of about twelve 

 species all containing much fat; green snow, principally caused by 

 zoogonidia of green algae; brown snow or ice due to the presence of 

 Mesotaenium and mineral matter; black snow caused by Scotiella 

 nivalis and Rhaphidonema brevirostre; and a hght, brownish-purple 

 ice-bloom caused by a species of Ancyclonema. 



West (191 6) divided the Aquatic Communities into the following 

 four major subdivisions, each of which can be further subdivided 

 in the manner illustrated above : 



I . Communities of rivers, rapids, and waterfalls 



In rapidly flowing rivers this is mainly composed of fresh-water 

 Rhodophyceae, e.g. Lemanea, Compsopogon, Sacheria, Thorea, 

 Hildenbrandtia (in acid waters, Batrachospermum and Bostrychia), 

 Cladophora spp., Vaucheria spp., Myxophyceae, Chrysophyceae 

 (especially Hy drums) and diatoms. Where the stteams are more 

 sluggish Enteromorpha and Hydrodictyon tend to replace Clado- 

 phora, and in very sluggish streams and also in lakes one may find 

 temporary floating mats of Hydrodictyon, Enteromorpha, Oedo- 

 gonium, Spirogyra and Rhizoclonium.^ Sluggish rivers also possess a 

 definite Potamoplankton divided into : 



^ This mostly refers to Europe. 



