452 



THE COMMUNITY 



operate jointly. The more important in- 

 fluences are listed by Welch (1935) as in- 

 cluding: (1) alteration of bottom, through 

 both mechanical stabiUzation by roots, and 

 retention of accumulating bottom deposits; 

 (2) mechanical support for hydras, sponges, 

 bryozoans, egg masses of insects, rotifers 

 and snails, insect larval cases, and many 

 algae; (3) breeding habitats, eggs being 

 laid both upon and in their tissues by many 

 insects; (4) reduction of hght; (5) tem- 

 porary shelter from predators; (6) reduc- 

 tion of wave action; (7) dispersal of 

 animals, by the breaking oS of leaves and 

 stems; (8) marl formation; (9) production 

 of dissolved oxygen, in excess of their 

 respiratory needs; and (10) consumption 

 of carbon dioxide. 



Where zonation of higher vegetation is 

 absent, as on the bare sand bottom of some 

 lakes, the apparently deserted strand above 

 the water Hne holds a diversified and 

 abundant fauna. The microscopic animals 

 composing this littoral zone (psammoUttoral 

 habitat) are subject to drastic fluctuations 

 of the physical environment. These include 

 two opposing movements of water: the up- 

 ward stream of capillary water, between 

 sand grains, rising from the lake and being 

 evaporated, and an intermittent, relatively 

 vigorous downward stream, from waves and 

 from rains. Within this hmited horizontal 

 stratum there is microstratification. The 

 mean water content for certain Wisconsin 

 lakes (Pennak, 1939) is 80 per cent satura- 

 tion in the top centimeter of sand at 100 

 centimeters from the water edge, 40 per 

 cent saturation at 200 centimeters, and 20 

 per cent at 300 centimeters. The width of 

 this strip is greatly influenced by slope. For 

 example, a sand beach with an 8 degree 

 slope has an aquatic populated zone 150 

 centimeters wide, while one with a slope 

 of 3 degrees may have this stratum 300 

 centimeters wide. 



Such a habitat supports an astonishingly 

 large biota (Fig. 156). An average 10 cc. 

 sample of sand (Pennak), taken 150 centi- 

 meters from the water's edge, will contain 

 between 2 and 3 centimeters of water, and 

 its fauna and flora consist approximately of: 

 4,000,000 bacteria; 8000 protozoans; 400 

 rotifers; 40 copepods, and 20 tardigrades. 



Of interest is the fact that even this 

 sandy beach, with its horizontal microstrati- 

 fication, is vertically microstratified by 



aqueous and evaporation gradients, and 

 further, that the animals are vertically 

 stratified within the top 8 or 10 centimeters 

 of sand. This vertical gradient is especially 

 well shown by the copepods and rotifers, 

 which usually occur with decreasing fre- 

 quency from the top centimeter downwards. 



Fig. 156. Diagram of abundance of mi- 

 nute organisms in the psammolittoral habitat. 

 I, Rotifers; 2, gastrotrichs; 3, tardigrade; 4, 

 nematodes; 5, harpacticoid copepods. (After 

 Pennak. ) 



The subhttoral zone is generally transi- 

 tional in character, of variable extent, and 

 typically lacks rooted vegetation, but has 

 much vegetable debris. Within this area 

 there is often a "shell zone," a belt char- 

 acterized by empty shells, and formed from 

 the thriving molluscan fife of the httoral 

 stratum. This shell zone is well developed 

 in the sublittoral of Lake Michigan and has 

 been found in numerous lakes of northern 

 Germany (Lundbeck, 1926) as well as in- 

 land lakes of Wisconsin, Indiana, and Japan 

 (cf. Eggleton, 1939). 



The true profundal zone is formed in 

 lakes that become thermally stratified. 

 Lakes of the second order, especially in 

 temperate regions, have this lowest stratum 

 developed. Conversely, lakes of the third 

 order are so shallow that no thermal strati- 

 fication occurs, and rooted vegetation may 

 transform the whole bottom into a littoral 

 zone. The profundal region is no more 

 self-supporting than the epilimnion above, 

 the several vertical and horizontal com- 

 partments being interdependent. 



From this point of view the vertical and 

 horizontal strata are not self-supporting, 

 whereas the pond or lake which they col- 

 lectively comprise is a relatively independ- 



