Zinc levels in the aquatic environment
The background concentrations of Zn in aquatic environments are comparatively low. In unpolluted areas, the concentrations of total Zn in the water have been reported to be 1 μg 1−1 or less (Spry, Wood & Hodson, 1981; Hogstrand, Lithner & Haux, 1991). Zinc has an extensive industrial use in alloys, galvanizing, pigments, and electrical equipment. On a relative basis, surface drainage and atmospheric fallout are the most important inputs of Zn to aquatic environments (Spear, 1981). Concentrations of waterborne Zn in industrialized areas rarely exceed 50 μg 1−1 (Coombs, 1980; Spear, 1981; Hogstrand and Haux, 1991), although concentrations of dissolved Zn far above 100 μg 1−1 have been reported (Abdullah et al., 1976; Roch & McCarter, 1984). There is also a coupling between acidification and increased waterborne concentrations of Zn, probably caused by leaching of Zn from rocks and sediments (Baker, 1982).
Zinc is a micronutrient
A total absence of Zn is not compatible with life. Although the involvement of Zn in biological systems has been suspected for a very long time, it was not until the middle of this century that evidence for the biochemical functions of Zn started to emerge. Keilin and Mann (1940) were the first to recognize the involvement of Zn in enzymes by their isolation of carbonic anhydrase. To date, over 300 proteins have been identified that need Zn for their functions and the number is rapidly increasing (Vallee & Falchuk, 1993). The biological activities of these proteins include steps in the metabolism of nucleic acids, proteins, carbohydrates, and fatty acids.