Plankton 433 



Schizothrix and Glaeocapsa, build up calcareous pebbles in the littoral region 

 of lakes (vide p. 35) and other Algae perforate both shells and stones. 



2. Plankton. The methods of collection of plankton-organisms and the 

 subsequent examination and estimation of catches, which are much the same 

 as in the case of marine plankton, have been well described by many authors 

 and the student is referred to the works of Apstein ('96), Kofoid ('97) and 

 Bachmann ('11). 



Three types of plankton are recognized : the LlMNOPLANKTON' of lakes, 

 the POTAMOPLANKTON of rivers 1 , and the CRYOPLANKTON of perpetual snow 

 and ice. 



Owing to the limited size of freshwater basins the shore- and bottom- 

 species play a more important part in the plankton than is the case in the 

 sea. In general, fresh waters contain a greater mass of plankton in the same 

 volume of water than does the sea, and, moreover, the plankton is of a more 

 composite character. 



LlMNOPLANKTON. 



The phytoplankton of lakes may be of great bulk, but more often it is 

 not. On the whole, it is unusual for it to colour the water to any appreciable 

 extent, except in shallow lakes on the more recent geological formations 2 . 

 The greatest amount of phytoplankton occurs in most cases during the 

 autumnal decline in temperature and in the great majority of lakes there are 

 certain more or less well-marked phases in the phytoplankton, each phase 

 dominated by one or more of the constituents. 



There is a considerable uniformity in the phytoplankton of lakes which 

 are situated on relatively recent geological formations, especially those of 

 lowland areas. On the other hand, the phytoplankton of the more upland 

 lakes of the older geological formations is usually quite different and, 

 moreover, affords striking contrasts even between lakes in the same area. 

 In many lake-areas it is not an easy matter to compare the phytoplankton of 

 one lake with that of another, since the annual phases of one probably do 

 not correspond with those of the other. 



Wesenberg-Lund ('08) has objected to Apstein's suggested grouping of 

 lakes according to the quality of the plankton, such as lakes with Myxophycea3 

 and lakes with Dinobryon. This objection was based, however, only on 

 a knowledge of the lakes of Denmark, which contain plankton of the most 

 monotonous character and are amongst the least interesting in Europe. 

 Apstein was fundamentally correct in his suggested classification of lakes 



1 Potamoplaukton only occurs in comparatively slow rivers, and is therefore discussed here 

 and not under the heading of 'Algal Associations of Swiftly-running Water.' 



2 Such colouration of the water is very noticeable in the shallow Danish lakes, in some of the 

 Baltic lakes, and in Lough Neagh and a few other Irish lakes. 



w. A. 28 



