15] VEGETATIONAL TYPES OF FRESH WATERS 483 



because of sinking, death, removal by currents, and consumption by 

 predators. 



In lakes there is commonly a marked and steep vertical gradient 

 of phytoplanktonic distribution, especially when the body of water 

 is limited in extent. For example, at the surface we may find an 

 almost continuous investment of often quite large plants such as 

 Duckweeds {Lemna spp.). Water-hyacinth {Eichhornia crassipes), and 

 the types shown in Fig. 164. Whereas these macroscopic plants 

 might be considered as belonging to the littoral, they often occur as 

 pelagials on ponds and small lakes, especially in warm regions. 

 Such relatively large floating material comprises the pleuston (hemi- 

 plankton), which is commonly defined as consisting of macroscopic 

 plants and as including those floating freely within the body of water 

 as well as on its surface. Furthermore, there are some microscopic 

 floating organisms (neuston) which stabilize their position upon the 

 surface of quiet water by employing surface-tension. For example, 

 a Green Alga, Nautococcus sp., becomes attached to the upper 

 surface-film by means of a flotation disk — thus essentially living as 

 an aerial organism, and forming conspicuous water-blooms of dry 

 (powdery) appearance. Other neuston organisms hang down in the 

 water, from the surface-film. 



Within the body of the water, most of the phytoplankton is usually 

 concentrated in the top 10 to 15 metres, its permanent survival being 

 limited to depths where more food material is made by photosynthesis 

 than is used in respiration over an average 24-hour period. This 

 maximum depth is dependent on light-penetration ; even in the 

 clearest alpine lakes the layer of water that is at all densely populated 

 by phvtoplankton scarcely exceeds 50 metres in thickness. Indeed 

 in the majority of lakes, at least in the higher latitudes, most of the 

 phytoplanktonic life is concentrated in the uppermost 5 metres 

 (according to Professor G. W. Prescott in lift.), while at depths below 

 about 30 metres the numbers of individuals decline to very small 

 values. Yet within these limits different species vary greatly in their 

 preference. Thus more than half of the forms are usually concen- 

 trated in the uppermost 10 metres or less, while others attain their 

 maximum concentration at or below a depth of 10 metres. Not a 

 few types, such as Cyanophyceae provided with gas-vacuoles, have 

 a specific gravity of less than unity and so are concentrated at the 

 surface. On the other hand some phytoplanktonic species can have 

 their maximum density at depths greater than 30 metres, an example 

 being the Diatom AsterioneUa formosa in some circumstances in 



