The effects of water hardness upon the uptake, accumulation and excretion of zinc in the brown trout, Salmo trutta L.

The effects of water hardness (9 and 220 mgl^?1 as CaCO₃) upon zinc exchange in brown trout exposed to 0.77 μmol Zn 1^?1 have been investigated using artificial soft water (<49.9 μmol Ca ^l-1, <40.1 μmol Mg 1^?1) and mains hard water (1671.7 μmol Ca 1^?1, 493.6 μmol Mg 1^?1) of known composition. Both hard and soft water-adapted fish exhibited a bimodal pattern of net zinc influx. Net zinc influxes during both fast and slow uptake phases were significantly greater ( P <0.001) in soft (82.9 and 6.2 μmol Zn 100 g^?1 h^?1) than in hard water (46.3 and 2.4 μmol Zn 100 g h^?1). Zinc efflux (- 0.2 μmol Zn 100 g^?1 h^?1) was enhanced only in hard water during the slow net influx phase.
Brown trout exposed to zinc in hard water and placed in metal-free media exhibited a greater net efflux (- 25.6 μmol Zn 100 g^?1 h^?1) of the metal than did fish in soft water (-4.2 μmol Zn 100 g^?1 h^?1) treated in the same manner. Tissue ⁶⁵Zn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 μmol Zn 1^?1) high water hardness reduced tissue burdens of the metal by reducing net branchial influx, and enhancing efflux of the metal in hard water fish.