order were the amphipod Hyalella azteca, the dip- 

 teran larvae Tendipes and Protcnthes, the snail 

 Amnicola. tubificid worms, and the sphaerid Pisidittm 

 (Eggleton 1952). For comparison, the depth distri- 

 bution of animals in an oligotrophic lake is shown 

 in Fig. 6-10. 



There is also a fauna of microscopic animals 

 inhabiting the bottom. This consists of Protozoa, 

 Hydra, Rhabdocoela (flatworm), Nematoda, Rota- 

 toria, Gastrotricha, Oligochaeta, Cladocera, Cope- 

 poda, Ostracoda, Acarina (mites), and Tardigrada. 

 These organisms are often very numerous in the thin 

 organic ooze-film that covers mud bottoms (Bigelow 

 1928), but may penetrate underlying deposits to 

 depths of 20 cm. Sand bottoms also support a varied 

 and abundant microfauna (Pennak 1940, Cole 1955). 

 In general, number of microfauna species and indi- 

 viduals varies inversely as the depth of water ; only 

 a few species remain active in the profundal zone dur- 

 ing the summer stagnation period (Moore 1939). 

 Bacteria, a source of food, are abundant in the bottom 

 at all depths. 



Much of the bottom fauna of the littoral zone 

 consists of immature stages of otherwise terrestrial 

 insects. The pulmonate snails and water mites have 

 evolved from terrestrial species. Other aquatic spe- 

 cies, however, have related forms in the sea and this 

 may indicate their evolutionary origin. The funda- 

 mental problem involved in dispersal from the sea 



FIG, 6-9 Two types of bottom samplers for measurement of 

 benthic populations. When open, each sampler covers a known 

 area. As each is closed, it scoops up the organisms present. 

 Right, Peterson's bottom sampler for hard or sandy bottoms; left, 

 Ekman's bottom sampler for soft bottoms and deep water (from 

 Welch 1948). 



into fresh water would be that of osmoregulation, and 

 the ability to live in fresh water has doubtless con- 

 stituted a selection factor in the origin of this com- 

 munity. 



I'rofundal zone 



In oligotrophic lakes, species characteristic of 

 the littoral zone are found at much greater depths 

 than they are in eutrophic lakes, in which the oxygen 

 supply during the summer stagnation period is re- 

 duced. The amphipod Pontoporeia occurs only in the 

 deeper oxygenated cold waters of some northern lakes 

 (Adamstone 1924). It is a relic from the glacial 

 period, when it was probably more widely distributed. 

 The profundal benthos of one oligotrophic lake in 

 British Columbia increased from 470 individuals per 

 m~ in January to 1270 in August (Ricker 1952). 



The most common bottom organisms are the an- 

 nelids Tubifex and Limnodrilus , and the insect lar- 

 \ae Tendipes. Chaohorus, Protenthes. There may 

 be a few moUusks, Pisiditim, and Musculium for in- 

 stance, nematodes, and other forms, including a 

 microscopic fauna (Eggleton 1931). 



The midge larvae represent a variety of species. 

 Fifty species occur in one small lake in Algonquin 

 Park, Ontario, that has a pH range of 4.6 to 6.6 

 and thermal stratification in summer with ample oxy- 



Lakes 7 1 



