196 



ANALYSIS OF THE ENVIRONMENT 



creasing part in hydrobiology and soil 

 science, since the gradient of oxidation- 

 reduction from top to bottom of a body of 

 water should be of value in relating such 

 factors as pH, oxygen, carbon dioxide, hy- 

 di-ogen sulfide, and ferrous and ferric iron. 



This is not to suggest such a prominent 

 role for redox potentials as was once 

 claimed for pH, but simply to greet with 

 satisfaction another index of organization of 

 the environmental background and its effect 

 on the organization of the community. Ac- 

 cording to expectation, stratified lakes have 

 a stratification in oxidation-reduction poten- 

 tials." A low content of dissolved oxygen 

 seems not to be the only factor involved in 

 decreasing the redox potential in the hypo- 

 Umnion; ferrous iron, hydrogen sulfide, and 

 organic reducing systems are also involved. 

 In oligotrophic lakes (those with relatively 

 poor nutritive supply) there was either no 

 decrease or only a sfight decrease in redox 

 potential of hypolimnial water; in eutrophic 

 lakes (those rich in nutritive materials) the 

 decrease in oxidation-reduction potential 

 was greater (Allgeier, Hafford, and Juday, 

 1941). 



The quantity of ferrous iron in solution 

 has been shown to determine the value of 

 the redox potential for certain lakes in Con- 

 necticut and New York (Hutchinson, 

 1938). Redox potentials and ferrous iron are 

 in close correlation with the occurrence of 

 larvae of several genera of Chironomidae, 

 often used as indicators of the trophic con- 

 ditions in the hypohmnial region, and Raw- 

 son ( 1939 ) believes that the redox potential 

 will provide a useful index for the habita- 

 bility of hypolimnial and benthic environ- 

 ments. 



HYDROGEN SULFIDE 



The deeper waters of lakes or ponds or 

 of isolated lagoons, bays, and fiords may 

 contain enough hydrogen sulfide to exclude 

 all Hfe except anaerobic bacteria. Small 

 ponds with a bottom of deep muck that 

 has a high organic content may also con- 

 tain much hydrogen sulfide in, or just 

 above, the bottom material. This poisonous 

 compound may also accumulate under ice 

 in winter, in the hypolimnion of thermally 



" Methods for measuring oxidation-reduction 

 potentials, matters of general theory in this 

 field, and general applications are discussed 

 in Michaelis (1930) and Hewitt (1937). 



stratified lakes or fiords in summer, and in 

 streams that are heavily contaminated by 

 sewage. Hydrogen sulfide is Hkely to occur 

 in ponds, lakes, or embayments of the sea 

 in which stagnant water underlies a rich 

 surface biota. The accumulations may be 

 local or may be geographic in extent, as in 

 the Black Sea (p. 193), where a shallow 

 "sill," provided by the Bosphorus Ridge, 

 that reaches to within some 40 meters of 

 the surface, prevents the renewal of the 

 deeper salt water by cutting off the Medi- 

 terranean circulation. Inflowing water tends 

 to float near the surface, and the lower 

 1900 meters have their dissolved oxygen 

 replaced by dissolved hydrogen sulfide; only 

 the upper 200 meters are aerated. 



Water rich in hydrogen sulfide has a low 

 redox potential. Not only are aerobic organ- 

 isms excluded from such waters, but 

 changes are produced in the physical en- 

 vironment. For example, iron, if present, is 

 precipitated as ferrous sulfide, and may 

 eventually become the relatively stable min 

 eral known as pyrite or fool's gold. 



CARBON DIOXIDE 



Carbon dioxide is dissolved in water as 

 a free gas, as are oxygen and nitrogen. Un- 

 hke these associated gases, carbon dioxide 

 also enters into chemical combination with 

 water to form the weak carbonic acid, 

 H2CO3, and by chemical reactions with 

 available alkalis, it forms half-bound and 

 bound carbonates, hence its solution in 

 lime-rich fresh water and in sea water does 

 not follow the usual gas "laws." The role 

 of these carbonates in buffering water 

 against rapid changes in pH has been dis- 

 cussed (p. 173), and other general effects 

 of the carbon dioxide content of water have 

 also been indicated (p. 76). 



Carbon dioxide enters water from the air, 

 from the ground, especially by means of in- 

 flowing ground water, from the decomposi- 

 tion of organic matter, from the respiration 

 of animals at all times and of plants in the 

 absence of fight, and from the action of 

 acids on bound and half-bound carbonates 

 dissolved in the water. In Hghted waters 

 this gas is removed by green plants in pho- 

 tosynthesis, in which process it is a basic 

 ingredient. The carbonates may be depos- 

 ited as marl, as the remains of calcareous 

 algae or as shells, especially of foraminif- 

 erans, corals, and moUusks. All these rela- 

 tions often set up and maintain a vertical 



