Lakes are large Ijodies of fresli water, often deep 

 enough to have a pronounced thermal stratification 

 for part of the year. Typically, shores are barren and 

 wave-swept (Muttkowski 1918). 



I.akes are formed in youthful stages of river 

 system develojjment. Water from upland runoff, 

 groundwater seepage, springs, and melting snow- 

 fiekls and glaciers collects in basins. As the basins 

 fill to overflowing, erosion of outlets starts ; as it goes 

 on, outlets are deepened and water level of the lake 

 drops. Products of erosion, carried into the basin by 

 wind and water, and the products of animal and plant 

 decay accumulate, making the water shallow. 



IVIorijhometry aside, the essential distinction be- 

 tween lake and stream habitats is the characteristic 

 of water movement ; continuous, rapid flow is the 

 characteristic of the stream, the lotic habitat. The 

 lake is a Icntic habitat ; the water is essentially a 

 standing, quiescent body, although at times wind ac- 

 tion stirs surface layer and margins into great turbu- 

 lence. Habitat factors associated with the lentic en- 

 vironment are uniquely modified to it (Welch 1948, 

 Hutchinson 1957). 



HABITAT 



6 



Local Habitats, 



Communities, and 



Succession: 



Lakes 



Pressure, density, and buoyancy 



The pressure imposed on a lake-dwelling or- 

 ganism is the weight of the column of water above it 

 plus the weight of the atmosphere. Most lakes have 

 a ma.ximum depth of less than 30 meters : the Great 

 Lakes of North America vary from 64 to 393 meters 

 in depth. Crater Lake in Oregon is the deepest on the 

 continent, 608 meters (Welch 1952). Ma.ximum 

 pressures are much less than in the ocean, and or- 

 ganisms appear to adjust to them readily. The ab- 

 sence of animal life from deep water is ordinarily a 

 consequence of low oxygen supply, or low tempera- 

 ture, rather than pressure. 



The density of water varies inversely with tem- 

 perature and directly with the concentration of dis- 

 solved substances. Water is most dense at approxi- 

 mately 4°C. Water becomes progressively less dense 

 as it is cooled below -)-4°C; ice expands markedly 

 (i.e., becomes less dense) the colder it gets. It is 

 because the coldest water is at the surface in 

 winter that ice forms there, rather than at the bot- 

 tom. In summer, the coldest waters of deep lakes 

 are at the bottom. Dissolved salts increase the 

 density of water; the density of most inland water- 

 bodies' is much less than that of the ocean. When 

 great evaporation occurs in a lake having no out- 

 let, as in the Great Basin, the lake may come to 

 contain a higher percentage of salts than the ocean. 

 The few species capable of living in these very salty 

 lakes include some algae and Protozoa, the brine 

 shrimp Artemia gracilis, and the immature stages of 



59 



