THE OCCURRENCE AND DISTRIBUTION 

 OF FRESHWATER ALG^E 



FRESHWATER Algse are universal in their occurrence, no moist situation 

 being without some type of Alga. They are found on damp earth, rocks, 

 walls, tree-trunks, etc. ; they are met with in all kinds of running water, from 

 the torrent and the waterfall to the slowest river; but it is in the still waters 

 of pools and lakes that they exhibit the greatest diversity and attain their 

 maximum abundance. They occupy, therefore, very varied habitats, and 

 it is because habitat plays such an important part in both the occurrence and 

 distribution of freshwater Alg& that it is here made the basis of the treatment 

 of the subject 1 . 



The ecology of freshwater Algas is still so much in its infancy that a 

 satisfactory ecological summary is impossible with our present knowledge. 

 The following account of the occurrence and distribution of these plants is 

 based largely upon the author's wide experience of them during the past 

 twenty years or more, although the views of other authors have been care- 

 fully considered and reference is constantly made to them 2 . In view of our 

 limited knowledge of the factors controlling the distribution and periodicity 

 of freshwater Algas generalizations are well nigh impossible and certainly 

 very unwise. 



Special ecological terms have been avoided as far as possible, since in 

 dealing with freshwater Algge they are so easily mis-applied. Most of the 

 algal vegetation of fresh waters can be regarded as forming associations of 

 a more or less definite character, the peculiarities of which are the direct 

 result of habitat and the nature and amount of the dissolved salts in the 

 water. The latter is a factor of great importance and is to a large extent 



1 Some of the main features of this chapter were explained by the author in a lecture 

 delivered at University College, London, on Feb. 18th, 1915. 



2 Many of the conclusions put forward by various authors in recent years have been very 

 erroneous and do not stand the test of enquiry. This is mainly owing to deductions having 

 been made from very limited investigations and to want of precise knowledge of the Algae dealt 

 with. To make any reliable contribution to our knowledge of algal ecology it is essential that 

 the author should have an extensive grasp of the taxonomies of the various groups, since to 

 discuss the distribution and periodicity of genera, as is so often done, is in most cases quite 

 futile and merely leads to confusion. 



