I SEP I OCT I NOV I DEC | JAN | FEB | MAR | APR | MAY | JUN | JUL 



Seasonal plankton populations In western Lake Erie through a year (after Char 



I AUG I 

 dier 1940). 



ing of waves is most effective, and the sublittoral 

 zone, which extends from the lower Hmit of wave 

 action to the lower limit of rooted vegetation. Where 

 such vegetation is absent, the sublittoral zone may be 

 considered the bottom of the epilimnion down to the 

 thermocline. 



The projundal zone is the entire bottom below the 

 rooted vegetation, or commonly the bottom of the 

 hypolimnion. The boundary lines between the zones 

 are variable and change with the depth of the thermo- 

 cline. The open water of the lake above the bottom 

 is known as the limnetic zone. 



Littoral zone 



The bottom of the littoral zone may be rock, 

 cobble, gravel, sand, or mud. The muddy shallows 

 of protected bays may have considerable rooted vege- 

 tation ; they are essentially pond habitats. Differen- 

 tiation of species distribution is primarily between 

 the hard bottom and mud bottom habitats ; sand bot- 

 tom habitats are transitional (Table 6-2). Sand bot- 

 toms ordinarily have the lowest population of most 

 species e.xcept clams because they are unstable habi- 

 tats at best ; indeed, they are often destructive by 

 reason of the action of sand grains grinding on each 

 other (Rawson 1930, Krecker and Lancaster 1933, 

 Lyman 1956). A lake-bottom and a streambed of 



similar composition will contain many of the same 

 kinds of organisms because of the similarity in the 

 physical conditions of existence. The respective spe- 

 cies compositions, however, are often different. 



Oneida Lake in New York has an unusually high 

 mollusk population. Baker (1918) recorded 59 spe- 

 cies and varieties. It is interesting that most of them 

 occurred on mud and sand bottoms. The highest 

 populations were 1890 individuals per m- on mud at 

 depths less than two meters, and 1573 individuals per 

 m- on sand. On rocks and gravels there were only 

 656 individuals per m-. 



In eutrophic Douglas Lake, Michigan, bottom de- 

 posits in the littoral areas show zonation down to a 

 depth of about 18 m. Beginning at the shoreline, 

 there are belts of barren, wave-washed sand, muddy 

 sand, sandy mud, and deep-water soft black ooze, in 

 that order. The average number of macroscopic ben- 

 thic animals is large, varying in the different types of 

 bottom from 369 to 1178 to 3822 to 1713 per m^, 

 respectively. The abundance of animals is related not 

 only to the nature of the bottom but also to depth 

 and vegetation present. Where vegetation was scarce 

 there were only 162 animals per m-, but with increas- 

 ing density of plants from sparse to common to abun- 

 dant the population of animals rose to 1531, 2525, 

 and 4407 per m-, respectively. Vegetation was most 

 dense at depths of 7 to 14 m in mixtures of sand and 

 mud. Most abundant animal species in decreasing 



70 Habitats, communities, succession 



