COMMUNITIES IN OTHER INLAND WATERS 367 



however, are poor in oxygen because of the amount used in processes 

 of decay. 



As a result of the poverty of the waters in nutritive and mineral 

 salts, moors have a reduced flora, particularly as regards the higher 

 plants, and this reduction is cumulative from the low to the high 

 moors. The phytoplankton is also sparse and may even be wholly 

 absent in European waters; Welch, 28 however, for Michigan bogs re- 

 ports more phyto- than zooplankton. The abundant supply of plant 

 detritus furnishes food for countless small animals, so that the animal 

 plankton may be rather abundant. The presence of humus, however, 

 and the absence of lime, bring about a selection so that the fauna 

 of moors is generally poor in species, although it may contain large 

 numbers of individuals. 



The humus water fauna. — Protozoans do not suffer much under 

 the adverse peculiarities of the bog waters; rhizopods especially are 

 represented by numerous species. Rotifers likewise do not seem to be 

 much affected by humus substances; of 186 rotifers in Galicia, 114 

 arc also found in the humus waters. 29 Planaria and leeches, on the other 

 hand, are missing in the bog ponds and from the mountain brooks fed 

 by moor waters. 30 Water mites seem, as a rule, to be absent from such 

 waters; fishes are usually absent. Their resistance against humus 

 elements, however, varies; it is strong in the stickleback, present to 

 a moderate degree in the carp, and least in the pike and trout. To- 

 gether with absence of fishes there is an absence of larger mussels, 

 Unio and Anodonta, whose larvae, as ectoparasites, are dependent 

 upon fishes. 



The scarcity of lime in moor waters handicaps the development 

 of mollusks. They are often completely absent as in the Blindensee in 

 the Black Forest; 31 in other places dwarfed forms of a few species of 

 JJmnaea and Planorbis are found, larger species being absent. The 

 pearl mussel {Unio ?nargaritifer) is absent in the creeks of non-cal- 

 careous mountains near the mouths of even small tributaries from 

 moor regions. The pea mussels (Pisidium) are least sensitive, occur- 

 ring in the mud on the bottom with Sphaerium in smaller numbers. 

 In lowland moors which have been formed by the filling up of ponds, 

 a layer of shells is often found beneath the peat, showing that a rich 

 snail and mussel fauna was once present. 



On the other hand, the scantiness of lime in the moor waters is of 

 advantage to the peculiar pelagic cladoceran, Holopedium gibberum 

 (Fig. 79), which seems to tolerate only water poor in lime and is 

 widely distributed in such waters. The colonial, sessile rotifers of the 

 genus Conochilus are often found in association with Holopedium. 



