COMMUNITY ORGANIZATION: STRATIFICATION 



453 



ent unit. This is simply a more modern way 

 of saying that the lake is a microcosm 

 (Forbes, 1887); it is a major commmiity. 

 This generally accepted viewpoint is sum- 

 marized by Eggleton (1939, p. 123): "If 

 any one characteristic of lacustrine ecology 

 is more often apparent to the limnologist 

 than any other, it is this interdependency 

 of the physical, chemical, and biological 

 phenomena whose constant interplay 

 weaves a complex design in the fabric of 

 the life of inland waters." Still more re- 

 cently Lindeman (1942, p. 399), in an im- 

 portant paper dealing with the trophic- 

 dynamic aspects of lakes, reaffirms this mi- 

 crocosmic view and states: "A lake is con- 

 sidered as a primary ecological unit in its 

 own right, since all the lesser 'communi- 

 ties' mentioned above are dependent upon 

 other components of the lacustrine food 

 cycle for their very existence." 



The lower portion of the sublittoral 

 usually merges into the upper portion of the 

 profundal. Where the littoral areas are 

 sandy, the bottom may grade insensibly 

 from sand, muddy sand, sandy mud, to 

 the mud of the profundal region. Many lake 

 animals reach a population maximum, dur- 

 ing the summer, in a band termed the "con- 

 centration zone." which typically occupies 

 a belt in the lower sublittoral-upper pro- 

 fundal. This concentration zone is formed 

 (Deevey, 1941) as the result of two in- 

 fluences: (1) the upward migration of 

 characteristic profundal species, such as 

 those of Chaobortis (= Corethra) among 

 Culicidae, and (2) the increase in numbers 

 of chironomid larvae. 



As in the psammolittoral habitat, there 

 is evidence of vertical microstratification of 

 the microscopic fauna of profundal mud 

 (Lenz, 1931), and this zone is also the 

 place of origin for the imperfectly under- 

 stood diurnal migrations of corethroid larvae 

 (Tuday, 1921). 



The profundal region, when typically 

 developed, is in strong contrast with the 

 littoral zone horizontallv and with the 

 epilimnion vertically. Here water tem- 

 perature, li2;ht, and dissolved oxygen are 

 minimal, while water pressure and carbon 

 dioxide are maximal; many gases of de- 

 composition, such as hydrogen sulfide, and 

 usually an acid pH, typify the profundal 

 mud and the water fust above. One would 

 anticipate under such conditions that 



Liebig's "Law of the Minimum" would 

 operate with respect to dissolved oxygen, 

 since this necessary element is typically low 

 or absent in many profundal areas during 

 stagnation periods. To this end, the hemo- 

 globin of the chironomid "blood-worms" 

 may be supposed to allow these char- 

 acteristic profvmdal larvae to exist under 

 almost anaerobic conditions; i.e., this sug- 

 gests one of the adjustments to low oxygen 

 tension. 



In relation to generally adverse condi- 

 tions, the profundal zone may have a rather 

 large population, chiefly of arthropods and 

 mollusks, composed of small to moderate- 

 sized individuals of a relatively few species. 

 Lake Michigan's (Eggleton, 1937) pro- 

 fundal floor produces organic matter equiv- 

 alent to at least 20 kilograms of dry or- 

 ganic material per hectare; Lake Mendota 

 (Tuday, 1922) produces at least 33.000 

 individuals per square meter; and Third 

 Sister Lake (Eggleton, 1931) produces at 

 least 71,000 individuals per square meter. 



One of the characteristic features of the 

 profundal habitat is its great variation in 

 seasonal population density. Making allow- 

 ances for type of bottom, this variation is 

 best explained bv the rhvthmical emergence 

 of the predominant insects. These include 

 Diptera (Chironomtis. Chaohorus) . hvdrop- 

 tilid Trichoptera, zvgopterous Odonata. 

 Ephemeroptera, and Heteroptera. Manv of 

 these emigrate vertically and emerge from 

 the epilimnial surface to lead an aerial 

 existence as imagines: others emigrate 

 horizontally into the shallo\^'er waters of 

 the sublittoral and littoral before emergence, 

 giving a disproportionate emergence index 

 for these latter areas (Scott and Opdvke, 

 1941). 



Marine Major Community 



With respect to area, the horizontal zones 

 of seas bear the same relation to those of 

 lakes as do the vertical gradients of fresh- 

 water and salt-water communities to each 

 other. A schematized diagram of the sea 

 floor (Fig. 157) begins with a relativelv 

 narrow tidal zone. This zone supports 

 the littoral strata, and gradually, with 

 deepening water, passes into an extensive 

 sublittoral zone of the continental shelf. 

 This shelf terminates relativelv abruptlv, 

 usually in the vicinity of the 200 meter 

 line, where the ocean floor becomes 



