15] VEGETATIONAL TYPES OF FRESH WATERS 495 



may be restricted to a zone only a few centimetres high, but which 

 still tends to be well marked and already divisible into belts that are 

 inundated for varying periods in an average year and consequently 

 exposed to rapidly changing temperature and other conditions. Thus 

 in some central European lakes there is often an uppermost ' emersion 

 belt ' that lies above the water for usually more than half the year 

 and is coloured brown by the cyanophycean Tolypothrix distorta, 

 which, when dry, can endure temperatures up to 70° C. without 

 injury. Below is typically a ' surf-belt ' of brownish to reddish- 

 yellow, pea-like crusts several millimetres thick and extending to 

 just below the low-water line. This surf-belt is dominated by other 

 Cyanophyceae — particularly by Rivularia haematites which forms 

 hemispherical colonies interspersed with stratified layers of lime, and, 

 in the upper portion, by Calothrix parietina which resembles flat 

 spots of chocolate. Numerous other forms occur here where mois- 

 ture is plentiful, including whole hosts of Diatoms and invertebrate 

 animals. Below, the effect of waves becomes weaker with increasing 

 depth — often no more than 10 cm. below the lowest water-level — 

 and the crusts of Rivularia, etc., give way to thick grey-green sedi- 

 ments including precipitated ' lime '. 



We should mention also the psammon, the interesting community 

 inhabiting the wet capillary spaces of sand-bars and sandy shores. 

 This forms a zone as much as 2 to 3 metres wide, extending above 

 high-water mark up to the limit of capillary attraction, and cor- 

 responds with the edaphon of the soil but consists mainly of Protozoa 

 and Bacteria, with some Algae near the surface. 



The sublittoral, where waves are no longer effective, typically 

 supports a rich growth of plants and animals of many and various 

 forms which are, however, killed by even brief emersion. Frequently 

 the cyanophycean Schizothrix lacustris is dominant, characterizing 

 a zone that can extend downwards for several metres if the dominant 

 is not overgrown by filamentous Algae. Here are often freshwater 

 Sponges and other animals coloured green by symbiotic Algae such 

 as Zoochlorella. For the water temperature tends to vary little and 

 photosynthesis to be untrammelled, so an especially rich and varied 

 life develops— provided the substratum is suitable for attachment and 

 the composition of the water is not unfavourable. 



Deeper down, in the infralittoral, the appearance of the benthos 

 begins to change markedly at depths usually coinciding with the 

 thermocline, for here plant life is drastically influenced by the de- 

 creasing temperature and light-intensity. Noticeable in this twilight 



