of methods, type of water, or concentrations. The 

 authors of many of the papers dealing with zinc 

 toxicity have used various specific sublethal effects 

 as endpoints and there is no way to compare these 

 findings with other work. 



Since the concentration of calcium and mag- 

 nesium influences heavy metal toxicity, permissible 

 levels of heavy metals are dependent on the cal- 

 cium and magnesium concentrations. Certain stud- 

 ies with zinc (Moxmt, 1966; and unpublished work 

 of the FWPCA National Water Quality Lab., Du- 

 luth, Minn.) and cadmium indicate that for a 

 given calcium and magnesium concentration the 

 acute toxicity of zinc and cadmium increases 

 (TLn, concentration decreases) as pH is raised 

 from 5 to 9. This seems contrary to prevalent 

 opinion that metal toxicity is related to metal in 

 solution and that as pH increases (solubility de- 

 creases) the toxicity decreases. The reason for this 

 apparent contradiction is that conceptions con- 

 cerning the effect of pH are based on natural 

 waters in which pH does not vary independently 

 of calcium and magnesium concentrations, but 

 rather is closely related to it. In those cases where 

 this relationship has been studied, except for one 

 (Sprague, 1964b), the toxicity has increased with 

 an increase in pH. This concept also is consistent 

 with the work of Lloyd (1961b) and, more re- 

 cently, that of Herbert and Wakeford (1964) who 

 concluded that colloidal or flocculated, but sus- 

 pended, zinc exerts a toxic influence on fish. 



The significance of temperature and the cal- 

 cium-magnesium content on the toxicity of zinc 

 to plankton has been pointed out by Patrick (un- 

 published data). In these tests, a 50-percent re- 

 duction in growth of the population was used as 

 a measure. Results of these tests are summarized 

 as follows : 



Concentration in mg/l which reduce:, 

 growth of population by 50 percent 



Ca~Mg concentra- 

 tion— 44 mg/l as 

 CaCOi Nilzchia 

 linearis 



Ca-Mg concentra- 

 tion — 170 mg/l as 

 CaCOs Navicula 

 seminulum 



72 F- 

 82 F_ 

 86 F. 



-4.29 mg/l 4.05 mg/l. 



-1.59 mg/l 2.31 mg/l. 



-1.32 mg/l 3.22 mg/l. 



Palmer (1957) found that zinc dimethyl dithio- 

 carbamate (ZDD) inhibited growth of Micro- 

 cystis at 0.004 mg/l. A concentration of 0.25 mg/l 

 controlled all diatoms, 43 percent of the blue- 

 green algae, and 18 percent of the green algae. 

 The above evidence implies that permissible levels 

 of zinc cannot be related to the calcium-magne- 

 sium concentrations or to pH alone. 



Herbert and Wakeford (1964) described the 

 effect of salinity on the toxicity of zinc to rain- 



bow trout. Since zinc was most toxic to trout in 

 freshwater, it is assumed that concentrations which 

 are safe in freshwater will be safe for the salmonids 

 in brackish water. The maximum reported affect 

 of a reduction of dissolved oxygen from 6-7 mg/l 

 to 2 mg/l on the acute toxicity of zinc is a 50- 

 percent increase in its acute toxicity (Lloyd, 

 1961a; Pickering, in press; Cairns and Scheier, 

 1958a). Since 4 mg/l is the minimum permitted, 

 this effect is small in comparison to the differ- 

 ence between safe and acutely toxic concentra- 

 tions. The use of an application factor, there- 

 fore, should provide adequate protection. Simi- 

 larly, Herbert and Shurban (1963a) found that 

 the 24-hour TL^ for zinc was reduced only 20 

 percent for rainbow trout forced to swim at 85 

 percent of their maximum sustained swimming 

 speed. 



The effect of calcium and magnesium con- 

 centrations on the toxicity of zinc for plankton, 

 invertebrates, fishes, and their embryonic stages 

 is reflected in the spread of values reported as 

 toxic by many sources (Anderson 1950; Brungs, 

 in press; Cairns and Scheier 1957, 1958b; Grande, 

 1966; Herbert and Shurben, 1963a, b; Jones, 

 1938; Lloyd, 1961b; Patrick, personal communi- 

 cation; Pickering, in press; Pickering and Hender- 

 son, 1966a; Pickering and Vigor, 1965; Skid- 

 more, 1964; Sprague, 1964a, b; Sprague and 

 Ramsey, 1965; Williams and Mount, 1965; and 

 Wurtz, 1962). 



Recommendation: The relationship between calcium 

 and magnesium concentration, pH, and zinc toxicity is 

 confusing and the separate effects have been little 

 studied. Brungs (in press) has determined that '^oo of 

 the 96-hour TLm value is a safe concentration for con- 

 tinuous exposure. 



Copper: The same general considerations apply 

 to the determination of safe levels of copper as 

 apply to safe levels for zinc and the discussion of 

 copper will be based on the same assumptions. 

 From the published data, differences in species 

 sensitivity to copper appear to be somewhat greater 

 than for zinc (Anderson, 1950; Grande, 1966; 

 Herbert and Vandyke, 1964; Jones, 1938; Lloyd, 

 1961b; Mount, in press; Pickering and Hender- 

 son, 1966a; Sprague, 1964a, b; Sprague and 

 Ramsey, 1965; Trama, 1954; TumbuU, DeMann, 

 and Weston, 1954). Mount (in press) has found 

 that Yso of the 96-hour TL^ value is a safe concen- 

 tration for continuous exposure of fish. 



Bringmann and Kuhn (1959a, b) report that 

 0.15 mg/l copper is the threshold concentration 

 which produces a noticeable effect on Scenedes- 

 mus. Maloney and Palmer (1956) report that 0.5 



60 



