Exposure of brown trout, Safmo frurfa, to zinc under continuous flow conditions over 96 h showed that both water hardness and pH exert major influences on the toxicity of the metal. 96-h LC50 values for total zinc ranged from cO.14mg I-' in alkaline soft water (PH 8; lOmg I-' as CaCO,) to 3.20 mg I-' in acidic hard water @H 5; 204 mg I-' as CaCOJ. A variable reduction in zinc toxicity in hard water compared with soft water over the pH range 4 9 was attributed to high external calcium. Zinc toxicity was positively correlated with decreasing acidity over the pH range 5-7, the metal being most toxic at pH 8-9 where metal complexes predominate. Below pH 5 metal toxicity also increased, irrespective of hardness. Water hardness and pH interacted with zinc toxicity in a complex manner, apparently dependent on physical and chemical transformations of the metal, and as changes in uptake, detoxification and excretion by the fish.

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The 96‐h LC50 on brown trout Salmo trutta of a commercial iron (III) sulphate liquor, used for treating reservoirs to reduce algal growth, was 28 mg total Fe l−1 (0·05 mg soluble Fe l−1). The 96‐h LC50 for analar grade iron (III) sulphate was 47 mg total Fe l−1 (0·24 mg soluble Fe l−1). Lethal and sublethal exposure to both grades of iron resulted in accumulation on the gill, which appears to be the main target for iron toxicity. Greater iron accumulation occurred during exposure to commercial iron sulphate liquor. Physical clogging of gills and gill damage was seen during lethal and sublethal exposure to iron. Gill tissue analysis showed no evidence of iron uptake into gill tissues during lethal or sublethal exposure to iron. Iron did not accumulate in plasma of fish exposed to iron compared to controls. Respiratory disruption due to physical clogging of the gills is suggested as a possible mechanism for iron toxicity.

The effects of water hardness (9 and 220mg I-' as CaCO,) upon zinc exchange in brown trout exposed to 0.77 pmol Zn I-I have been investigated using artificial soft water ( <49.9 pmol Ca I-', ~4 0 . 1 pmol Mg I-') and mains hard water (1671.7pmol Ca I-', 493.6pmol Mg I -' ) 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 pmol Zn 100 g-I h-I) than in hard water (46.3 and 2.4 pmol Zn 100 g ' h-l). Zinc efflux (-0.2 p o l Zn 100 g-' h-') 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 pmol Zn 100 g-' h-') of the metal than did fish in soft water (-4.2 pmol Zn 100 g-' h-') treated in the same manner. Tissue bSZn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 pmol Zn 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.