COMMUNITIES IN STANDING WATERS 349 



The plant growth of the bottom produces sufficient amounts of oxy- 

 gen. Processes of warming and cooling take place more rapidly be- 

 cause of the depth of the water, and consequently the temperature of 

 the water fluctuates with that of the atmosphere. The great extension 

 of the bottom area in relation to the amount of water, other things 

 being equal, brings about an increase of the amount of material dis- 

 solved in the water. The consequent large development of the flora 

 results in a similarly more extensive development of fauna. The ani- 

 mal life of the smaller water basins is much like that of larger bodies 

 of water in number of species, but it is richer in the number of in- 

 dividuals. To the ponds which have their origin in the ageing of lakes, 

 may be added ox-bow lakes of rivers, and artificial ponds, because 

 of the characteristics of their biota. 



The communities of the small bodies of water are not unlike those 

 found in the littoral region of lakes (cf. p. 326 above) ; although they 

 differ in many particulars. There is no room here for strongly steno- 

 thermal animals; the flatworm Planaria alpina, for instance, occurs 

 in the deep waters of many lakes but not in small bodies of standing- 

 water. The relatively large extent of bottom in ponds, with its rich 

 supply of plants and detritus, is a veritable paradise for snails and 

 mussels. These grow to be much larger here, as a rule, than in lakes 

 (cf. p. 330). The abundant insect larvae of caddis flies, dragonflies, and 

 may flies belong to many different species. Many of the Crustacea of 

 the lakes are entirely missing in the small water basins, e.g., Bosmina 

 coregoni, Sida crystallina, and the copepods Diaptomus gracilis and 

 graciloides; others, on the other hand, occur here as well as in the 

 lakes.* 



There are very few, if any, of the rotifers of the lake plankton which 

 are not also found in the plankton of ponds. 69 The limited stretches of 

 open water reveal a floating fauna of a different composition of species 

 and varieties as compared with the open waters of lakes. Here one 

 often finds forms which in the lakes are limited to the shore region— 

 thus the rotifer Brachionus pala and many others and the cladoceran, 

 Ceriodaphnia. The rotifers show a noticeable mass development in 

 ponds, especially in the uppermost stratum; in July, in Bohemian 

 ponds, they constitute 90% of all plankton animals at the surface and 

 40%- at 1 m. 70 



Many species of animals take on different forms in the smaller 

 water basins from those shown in the lakes. The rotifer Triarthra 

 longiseta 7 ' 1 of the ponds is distinguished from the lake form by a 



* Daphnia longispina, Scapholeberis mucronala, Polyphemus pediculus, etc. 



