FRESH-WATER ECOLOGY 333 



divisions of the aquatic associations can be recognized (see below) 

 and each one of the subdivisions can be treated as follows : 

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



A B 



Permanently attached Temporarily attached 



■^'~ 



(a) On living material. 



(b) On dead organic material. 



(c) On inorganic material (EpiHthic). 



(d) In the silt. 



(2) Floating macro-communities (Pleuston) : 



(a) Originating from loose bottom forms. 



(b) Originating from epiphytic forms. 



(c) Wholly floating throughout. 



(3) Loose-lying communities of the bottom. 



(4) Plankton or floating communities : 



(i) Limnoplankton of lakes, 

 (ii) Potamoplankton of slow rivers, 

 (iii) 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 

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

 Mesotaenium and mineral matter; black snow caused by Scotiella 

 nivalis and Rhaphidonema brevirostre ; and a light brownish purple 

 ice-bloom caused by a species of Ancyclonema. 



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

 four major subdivisions, each of which can be further subdivided 

 in the manner illustrated above : 



(i) Associations of rivers, rapids, and waterfalls . 



This is mainly composed of fresh- water Rhodophyceae, Clado- 

 phora spp., Vaucheria spp., Cyanophyceae and diatoms, whilst the 

 flora of hot springs may also be included here. Cyanophyceae are 

 usually the only constituents, the various species being capable of 

 secreting carbonate of lime or silica to form rock masses such as 

 travertine and sinter, the rate of deposition sometimes being as much 

 as I -25-1 '5 mm. in three days. The highest temperature recorded 



