368 ANIMALS IN INLAND WATERS 



The zooplankton of the moor pond is occasionally plentiful, as 

 compared with the phytoplankton ; it feeds upon the abundant plant 

 detritus. Copepods, cladocerans, and rotifers may be found in great 

 abundance. Larvae of the gnat, Corethra (cf. p. 350) , stay in deep wa- 

 ter in larger basins during the day, but also occur ubiquitously in the 

 smaller ditches and puddles containing bog waters. 



The adverse influence of humus extracts on aquatic life in general 

 is clearly shown in the black water rivers of the Amazon region, the 

 Coary, Teffe, Jurua, and Rio Negro, where the mosquito larvae do 

 not seem to thrive, so that cities like Coary and Ega are welcome 

 places of refuge for travelers. 32 



The deeper parts of the moor waters are poor in animal life or 

 occasionally entirely uninhabited because of the prevailing scarcity 

 of oxygen. In the Scottish lochs the most prevalent inhabitants of the 

 deeps are the larvae of Chironomidae ; in addition to these there are a 

 few Tubifex worms, ostracods {Cypris), and pea mussels {Pisidium) , 33 

 It is unusual to find more than 10 to 20 individuals on a square meter 

 of bottom. The appearance of ostracods is erratic, for they occur in 

 great numbers in high moors in the Alps and in those of the Sarek 

 Mountains in Norway, but may be completely absent in other moor 

 waters. 



The temperature conditions of many moor waters are distinctive. 

 Temperature variation is much reduced by the moss; the daily and 

 annual variation of temperature is diminished; the minimum and 

 maximum temperatures are transmitted very slowly to the deepest 

 layers, so that the moor springs are often higher in temperature in 

 winter than in summer. For this reason many moor waters are places 

 of congregation for stenothermal cold-water animals such as the 

 copepod, Canthocamptus arcticus, which can stay there along with 

 ubiquitous eurythermal forms. They are often spoken of as a relict 

 fauna of the glacial age because they frequently occur simultaneously 

 in north Scandinavian and in alpine waters. Though relatively little 

 studied in America, Michigan bog-lakes are known to contain all the 

 rotifers listed by Harnish 34 for European moor lakes together with 

 many of the same plant plankters and crustaceans. 28 



Animal communities associated with brine. — Waters with 

 strong salt content can maintain the amount of salt in various ways. 

 In one case springs bring up water rich in salt from the deeper layers 

 of the earth and either pour this into larger rivers where the salt con- 

 tent is greatly reduced by dilution, or else they pour it into swamps, 

 ponds, or lakes with no outlets, in which the concentration of the salt 

 solution is then increased by evaporation. In arid regions, lake basins 



