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0011 Dissolved Oxygen ID.O.I Oxygen is highly soluble in water that is under- saturated, and the amount of oxygen dissolved depends on temperature. salinity. and atmos- pheric pressure. The primary sources of oxygen in a stream are physical aeration and photosyn- thesis (Cole, 1979 1281 I. Physical aeration by turbulence is the most important source of dissolved oxygen in running water (Hynes, 19701 1841. Most mountain streams are tur- bulent and have a dissolved oxygen concentra- tion near saturation (Reid. 1961 11281). Ox- ygen from photosynthesis contributes to the dissolved oxygen content during the day, but photosynthesis is dependent upon the amount of sunlight and the quantity of vegetation. pH The pH expresses the intensity of an acid as well as the total amount of acid that is present (Cole, 1979 1281). The pH level. in addition to being potentially harmful in itself, is related to the con- centration of many other substances. particular- ly the weakly dissociated acids and bases (McKee and Wolf, 1963 1105] I. Ellis (1937) 147] reports that most of the inland waters of the United States that contained fish have a pH between 6.7 and 8.6. Conductivity Specific conductance, or conductivity, in micro- siemens per centimeter (I'Slcm), is the reciprocal of the specific resistance of a solution measured between two electrodes 1 cm' in area and 1 cm apart. The conductivity of natural waters .is usually closely proportional to concen- trations of the major ions (Juday and Birge, 1933 IB91. and Rodhe, 1949 11311), Measurement of conductivity is a rapid method for determining the ion concentration of water and the resulting osmotic pressure on aquatic organisms. Hart et al. (194511711 reported that 95 percent of the United States waters support- ing a good fish fauna had a conductivity of less than 1100 I'Slcm. Oxidation-Reduction Potential (Eh) Oxidation-reduction potentials IEhl in natural waters are usually expressed as their activities in neutral water (pH 71. Thus, these Eh values are expressed as E" in which the correction is made that adjusts all samples to a pH of 7 (Wetzel. 1975 11651 I. True oxidation- reduction equilibrium is not found in natural aquatic systems because of the extreme slowness of most reactions and the continuous state of change caused by temperature fluctua- tions and continuous cycles of photosynthesis and respiration causing changes in pH and oxy- gen content (Stumm. 1967 11431 I. Equilibrium would seem even less likely to occur in a lotic system. However, shifts in oxidation-reduction potential in profundal waters have been shown to have strong implications for chemical interac- tions at the sediment-water interface (Hutchin- son, 1957 1811; Hargrave, 1972 [68/; and Cole. 1979 1281 ). According to Brannon et al. (19781 1131. organic oxidation-reduction systems. in addition to inorganic oxidation- reduction systems. can influence Eh. Mortimer (19421 11141 states that an E, of 200 mV at the sediment-water interface is often considered the transition from an oxidizing to a reducing environment. Under anaerobic conditions, some species of bacteria use electron acceptors other than oxy- gen. It is generally accepted that nitrite is the first electron acceptor following the depletion of oxygen, followed by manganese and iron ox- ides, sulfides. and carbon dioxide (Golterman, 1975 1661 and Van Kessel. 1978 115511. The operating manual for an electronic probe which measures oxidation-reduction potential states that: "At the present time, oxidation- reduction measurements in natural waters can be interpreted only qualitatively. That is. no general approach to quantitative interpretation is known because of the complexities of natural water chemistry and the fact that many oxidation-reduction reactions occurring in natural waters do not reach equilibrium," (Hydrolab Corp., 1972 [82] ). Chemical Parameters Many chemical and physical parameters of water. when considered in total, provide what is referred to as "water quality." The levels of various parameters dictate the types and numbers of aquatic organisms that are able to survive in a particular habitat. Liebig (1 840) 1971 and Shelford (1913) [1381 formulated principles which explain biological yield as it 9