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0012 principal anion in fresh waters. It is an important component of sulfhydryl bonding in protoplasm (Cole. 1979 [28J I. In waters of the United States that contain good fish populations. 95 percent have sulfate values of less than 90 mg/L (Hart et aI., 1945 (711). Chloride Chloride (CI) is a constituent of rain and snow throughout the world and is fairly uniform within each locality (Hutchinson, 1957 1811), Other sources of chloride include waters containing chloride leached from igneous rock, releases from chlorinated domestic sewage treatment plants ICole. 1979 (281), and runoff from winter salting of highways (Environmental Pro- tection Agency, 1971 [501 l. Hart et al. (1945) [711 report data which indicates that 95 per- cent of United States waters with good fish populations have chloride concentrations below 170 mg/L. Adams (1941) 111 reported that a concentration of 400 mg/L was harmful to trout. Phosphorus Phosphorus IPI is an extremely important ele- ment for primary production in streams, and because of sources in rainfall, runoff. and domestic sewage discharges. it is seldom a limiting factor for plant growth in a natural stream IHynes, 1970 [84J and Golterman. 1975 (661). Phosphorus is essential to all life in that it is a component of A TP (adenosine tri- phosphate), which is instrumental in providing cellular energy. The inorganic form. orthophos- phate, is highly reactive and can be used im- mediately by autotrophic plants ICole, 1979 [281 I. Phosphorus is highly concentrated in aquatic food chains. A study by Davis et al. 119581 [351 found concentrations of the radio- active tracer phosphorus-32 to be 370,000 times higher in the caddisfly larvae. Hydrop- syche. than in the water. Nitrogen Nitrogen IN). like phosphorus, is an important component for primary production in aquatic systems. Nitrogen usually exists as ammonia, (NH,), nitrite INO,). nitrate INO,). and organic nitrogen ITKNl. Nitrate is the most available form for plant growth, and this fully oxidized form occurs most often in running water (Hynes. 1970 [841). A variety of sources such as sewage and other forms of pollution, and the abundance of nitrogen in the atmosphere, en- sures that nitrogen seldom limits plants produc. tion in aquatic ecosystems ICole. 1979 [281 l. Hart et al. (1945) [711 report that 95 percent of the waters in the United States that contain good fish fauna have nitrate concentrations of less than 4.2 mg/L. Heavy Metals The heavy metals referred to in this review in- clude manganese IMn). iron IFe). copper ICul, zinc IZn), lead IPbl, molybdenum IMo). and cad. mium ICd). Wood (1975) [1731 has classified metals into three categories: 111 noncritical. (2) toxic but very insoluble or rare, and 13) very toxic and relatively abundant. IMetals of atomic number less than 40 are generally abundant and usually cause the greatest concern in relation to aquatic ecosystems.) Many of these elements are needed in small con- centration for primary production and are there- fore essential for all life IFogg, 1975 [571 I. Toxicity results when these elements reach levels that can no longer be tolerated or avoided by aquatic organisms. There have been many literature reviews of the toxicity of heavy metals to aquatic organisms (McKee and Wolfe. 1963 [105]; Environmental Protection Agency, 1971 [491; and Becker and Thatcher, 1973 [7J l. Much of the literature deals with acute toxicity levels, with less em- phasis on the study of safe levels for chronic ex- posure. Concentrations of heavy metals in waters are generally higher in summer than in winter INam- minga and Wilhm. 1977 [1181 I. This is prob- ably due to an increase in microbial activity in the warmer sediments and a corresponding in- crease in metal concentrations I Duke et al.. 1966 [411 I. Under conditions of low dissolved oxygen. pH. and oxidation-reduction potential, heavy metals can be released into the water (Sartoris et al., 1977 [133] I. Mathis and Cum- mings (1971) [104J showed that organisms that inhabited the mud-water interface had high levels of metals in their body tissues; however, fishes that were primarily carnivorous or higher in the food chain had the lowest concentrations. In contrast, other studies have shown a high 11