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compares to: (1) minimum requirements of cer- tain factors that would limit production, and (2) maximum limits that would result in a toxic effect. Hart and Fuller (19741 1701 provide a summary of the water quality requirements for the major 9roups of aquatic animals. Hardness Hardness, according to McKee and Wolf (19631 11051. is attributable principally to calcium and magnesium ions and is considered to be the soap-neutralizing power of water; however, other elements may also contribute to hardness. Hardness is an important parameter in heavy metal toxicity. In hard water, metal ions are precipitated, thus rendering them less toxic to aquatic organisms. Calcium An ample store of calcium (Ca) in the earth's crust contributes to formation of a predominant compound in most inland waters, calcium car- bonate. Some plants and animals can be classi- fied as either calciphiles or calcipholes, depend- ing on whether they are found in waters of high or low calcium content. Calcium is usually the major cation involved in the bicarbonate- carbonate system (Cole. 1979 [281 ). Calcium in water reduces the toxicity of many chemical compounds but could be toxic to fish at concen- trations of between 300 and 1000 mg/L (McKee and Wolf, 1963 11051 ). According to Geijskes (1935) [631 calcium carbonate deposits make the substrate unsuitable for many of the usual stream-dwelling orders or organisms such as Ephemeroptera. Plecoptera. and Trichoptera. Hart et al. (19451 [711 reports that 95 percent of the waters in the United States that support good mixed fish populations have a calcium content of less than 52 mg/L. Magnesium In temperate regions, the second most abundant cation in inland waters is usually magnesium (Mg). This is an extremely important element in photosynthesis because of its central position in the chlorophyll molecule (Cole. 1979 1281), Hart et al. (1945) [711 report that 95 percent of all United States waters supporting a good fish fauna have a magnesium concentration of less than 14 mg/L. Sodium The active element sodium (Na) is abundant but does not occur in a free elemental state in nature. Most sodium salts are extremely soluble in water (McKee and Wolf, 1963) [1051. Low sodium concentrations are typical of waters draining from many montane areas (Hutchinson, 1957 [8111. Sodium is an important compo- nent of cellular fluids and active transport (Cole, 1979 [281 I. Potassium Potassium (KI. like sodium, constitutes a large portion of both the extracellular and intracellular fluids of plants and animals and is involved pri- marily in ion transport and exchange (Wetzel, 1975 [1651 ). This monovalent cation is com- mon in the earth's crust, reacts vigorously with oxygen and water. and is therefore not found in nature in the free state. Several studies have shown that potassium can be toxic to fish in soft waters at concentrations of 50 to 200 mg/L, but for lower organisms, such as Trichoptera nymphs, the toxicity level is 1000 mg/L of potassium (McKee and Wolf. 1963 [1051). lit- tle information is available on potassium toxicity to aquatic organisms in hard water. Carbon Dioxide Although carbon dioxide (CO,I is found in very small quantities in the atmosphere. it is relatively abundant in natural waters. Most CO, in aquatic systems has its source in rainfall or the dissolu- tion of carbonate bearing sediments (Cole. 1979 [281), Within the aquatic system it cycles rapidly and is constantly being used dur- ing plant photosynthesis and produced during plant respiration. The pH of the water mediates the form of CO, and its availability to plants. At a pH of greater than 8.3 the shift is from a bicar- bonate (HCO,) to a carbonate (CO,I form and no measurable free CO, is available for plant use (Reid, 1961 [1281), Sulfate Sulfate (50,) is present in rainwater due pri- marily to air pollution. This, and the leaching of sulfur-bearing sediments or organic deposits, provide the major sources of sulfur for an aquatic ecosystem (Reid. 1961 [1281), The sulfate ion is usually second to carbonate as the 10