COMMUNITY ORGANIZATION: STRATIFICATION 



443 



quirements are satisfied in this community. 

 These are themselves of diverse ranks and 

 may be of temporary or permanent resi- 

 dence. Such tertiary inhabitants would in- 

 clude athlete's foot fungus, dog and cat 

 fleas and lice, human lice, numerous patho- 

 genic bacteria and viruses, dysentery 

 amebae, as well as numerous pets brought 

 into the community by the human compo- 

 nent. 



Such established organismal gradients 

 are both quantitative and qualitative, and 

 the integration can be detected and meas- 

 ured by direct examination of the environ- 

 mental gradients with inferential data con- 

 cerning the organismal stratification; or the 

 latter can be studied and environmental re- 

 lations deduced, or, preferably, both envi- 

 ronmental influences and the biological in- 

 dicators may be utilized at the same time. 

 It is perhaps unfortunate that data obtained 

 from this last plan of study cannot be 

 presented here in their natural unity. Such 

 a combined treatment would require repe- 

 tition in this section of details of physical 

 stratification that have been given appro- 

 priately at some length in preceding sec- 

 tions. 



VERTICAL STRATIFICATION IN 

 AQUATIC COMMUNITIES 



Vertical stratification is well developed 

 in many aquatic habitats. These show 

 readily detectable vertical gradients in tem- 

 perature (p. 93), light intensity (p. 449), 

 wavelength absorption (p. 124), other 

 phvsical conditions, including distance from 

 surface and substrate (p. 158), dis- 

 solved chemicals (p. 198), pU (p. 172), 

 redox potential (p. 195), and dissolved 

 gases (p. 193). Thermal stratification in 

 summer and winter stagnation under ice, 

 together with the intervening periods of 

 ventilation produced by vernal and autum- 

 nal overturns, occur throughout the world 

 in temperate lakes of the second order (p. 

 95). 



With these points concerning the gra- 

 diented environmental background of fresh- 

 water communities in mind, a little-under- 

 stood, but essential, group of gradiented 

 influences deserves attention. Up to this 

 point organic materials have been brought 

 in obUquely, as in the decomposition of or- 

 ganisms falling from epilimnial to hypo- 

 limnial strata. We have been more directly 



concerned with discussion of gradients of 

 such physical influences as temperature and 

 light, or hydrogen ion concentration and 

 redox potential, or inorganic salts of iron, 

 calcium, phosphorus, and nitrogen, or gas- 

 eous oxygen, carbon dioxide and the car- 

 bonates and bicarbonates, and hydrogen 

 sulfide. 



Organic materials have two sources com- 

 monly recognized in lakes, but not easily 

 separated as to their eS"ects. In the first 

 place, there are the allochthonous organic 

 materials, derived from the external terrain 

 by seepage, or carried into the lake by 

 drainage. Second, there are the autoch- 

 thonous materials, produced within the 

 lake; that is, they are of internal origin and 

 are derived from the decomposition of the 

 bodies of the organisms living in it. These 

 latter materials deserve special notice since 

 they are more direct products of the com- 

 munity and in great part determine its self- 

 sustaining capacities. Autochthones are de- 

 rived primarily from the epilimnion, since 

 the bottom organisms, although they add 

 their own bodies to the total organic po- 

 tential, are more or less dependent upon 

 the regular increment from above. Such 

 organic materials, regardless of their exter- 

 nal or internal origin, and regardless of 

 their original stratal position, tend to accu- 

 mulate in the deeper levels of the hypolim- 

 nion and build up the bottom materials. 

 These organic particles diminish in size, 

 with progressive decomposition, and grad- 

 ually, through complex stages involving 

 oxidation-reduction systems and biological 

 action of bacteria, produce inorganic 

 components, or unite with external radicals. 

 One of the significant end products is the 

 building up and seasonal dispersal of raw 

 materials that may be used in future pro- 

 tein synthesis, such as nitrates and phos- 

 phates. The reactions taking place are 

 chiefly the cause for oxygen deficiency and 

 other typical hypolimnial features, so that 

 it should be kept in mind that the lake or- 

 ganisms in death are as important to the 

 future of the community as are the living 

 organisms. 



As lakes mature with age, there is usually 

 an increase in total vegetation, resulting in 

 concomitant increase in organic materials, 

 from phytoplankton and phanerogamic 

 plants, and from the associated herbivores, 

 carnivores, and saprovores. Increase in or- 



