444 



THE COMMUNITY 



ganic supply plays its part in filling up the 

 hypolimnion, together with inorganic sedi- 

 ment, so that study of organic materials is 

 desirable for both a present view of com- 

 munity mechanics and for a clearer under- 

 standing of community development. 



As organic materials settle in the hypo- 

 limnion they become the focal point of 

 complex dynamic influences, among which 

 bacteria are notable agents. These mate- 

 rials, by selective settling out and preserva- 

 tion, form a part of the lake bottom. Such 

 bottom deposits include silica (from dia- 

 tom shells), calcium carbonate, and organic 

 materials (Wilson and Opdyke, 1941). 



Organic materials comprise both dis- 

 solved and particulate portions. The dis- 

 solved organic materials (Birge and Juday, 

 1934) of Wisconsin lakes were shown to 

 comprise about 75 per cent carbohydrates 

 and 25 per cent proteins, with a trace of 

 fats. Birge and Juday found that the total 

 organic material, in lakes which were 

 largely autochthonous, ran about 4 mg. per 

 liter, of which 16 per cent was planktonic. 

 The average of all lakes they studied in 

 Wisconsin (autochthonous and allochthon- 

 ous) ran 16 mg. per liter of organic mate- 

 rials, with plankton forming 8 per cent. 

 This indicates that total organic materials 

 increase in allochthonous lakes, while 

 plankton-organic materials decrease, and 

 suggests a higher degree of productivity 

 in autochthonous lakes. This indicates that 

 such lakes support more closely balanced 

 and self-sustaining communities. 



If we assume with Rawson (1939) that 

 the amount of dissolved organic material 

 is about seven times as large as the amount 

 of plankton, then two questions arise: How 

 is this dissolved organic component made 

 available for protoplasmic svnthesis, and to 

 what extent is this material utilized? The 

 general view is that lake bacteria break the 

 dissolved materials into phosphates, ni- 

 trates, and ammonia, from which inorganic 

 dissolved compounds, phanerogams, and 

 phytoplankton build their protein. Our 

 ignorance here concerning many bacterial 

 and biochemical problems is large. 



Another view, less generally accepted, is 

 that planktonic plants and animals can 

 utilize dissolved organic materials directly 

 in their protein synthesis, without recourse 

 to the bacterial-inorganic portion of the 

 cycle. This is a broadened Piitter hypothe- 



sis, discussed previously (see Index). The 

 hypothesis is of interest here since it bears 

 upon the basic relationships of the food 

 chain within the aquatic communities (pp. 

 497, 500). Piitter (1909) postulated that 

 most small zooplankters derived much of 

 their nutrition from the dissolved organic 

 materials in water. This controversial hy- 

 pothesis is still stimulating research. Hasler 

 (1935) found that Daphnia magna were 

 able to digest protein and carbohydrate, 

 presumably as particulate food, through 

 the agency of an intestinal proteolytic 

 enzyme similar to trypsin. Gellis and 

 Clarke (1935) found that this clad- 

 oceran could derive nourishment from 

 colloidal organic matter. An intermediate 

 position was taken by Klugh (1927), who 

 found some entomostracans could utilize 

 fine detritus, but that their chief food was 

 phytoplanktonic green algae. Krogh (1930) 

 did not find dissolved organic substances 

 of importance in nutrition of aquatic ani- 

 mals, and (1931) concluded that, although 

 some utilization might occur, it was on too 

 small a scale to become important. Stuart, 

 McPherson, and Cooper (1931) raised bac- 

 teriologically sterile Moina and found them 

 unable to subsist on dissolved organic ma- 

 terial, and Bond (1933) found a similar 

 negative correlation. Clarke and GelHs 

 (1935), turning their attention to marine 

 copepods, found that their chief foods were 

 bacteria and other nannoplankton. 



From this summary we emerge with the 

 belief that we need a more comprehensive 

 knowledge of the role of bacteria in the 

 breakdown of organic materials dissolved 

 in water and the use of bacteria as food by 

 small aquatic animals, better methods of 

 assay, a rigorous application of techniques 

 to insure that experimental media are free 

 of bacteria, and a wider sampling of the 

 plankton. Until these precautions have 

 been widely applied, we may not com- 

 pletely discard Piitter's early assumption. 

 At present, the preponderant balance of 

 evidence is in favor of some utilization of 

 dissolved inorganic substances in nutrition 

 of animal plankton; although, as suggested 

 by Varga (1934), direct utilization of dis- 

 solved organic substances cannot be ex- 

 cluded. When more information has become 

 available, we may find that Piitter's hypoth- 

 esis is too limited in application to be 

 treated as a general factor in planktonic 



