Abstract-A standardized laboratory setup, simulating field mixing zones that originate at the confluence of limed rivers with acidic, aluminium-rich, tributaries, has been developed. Detailed analyses of the chemical speciation of aluminium (Al) in relation to the biological response of brown trout were performed to identify the mechanism of unexpected high fish mortality in the above-mentioned mixing zones with pH levels above 6.0. Brown trout experienced an acute cumulative mortality (98% in 48 h) immediately after neutral and acidic, Al-rich, water had mixed. Mortality was only 60% within 48 h of exposure to the acid water with Al (pH 4.6 ϩ 6.8 mol Al/L), although the Al concentration in the mixing zone was less (2.8 mol Al/L) at a nonharmful pH level (pH 6.4). Chemical speciation and ultrafiltration studies demonstrated that the transformation of low-molecular weight Al (Ͻ10 kD) into high-molecular weight Al (Ͼ10 kD), defined as Al polymerization, could better be related to the toxic response of fish, than the total Al concentration. The aging of polymerized Al forms (for 480 s) resulted in reduced fish mortality and less pronounced physiological stress. Brown trout in the initial mixing zone showed significantly increased plasma glucose and cortisol levels. Light and electron microscopy studies demonstrated serious damage to the skin: increased mucous secretion, a high ratio of acid to neutral glycoprotein-containing mucous cells, increased apoptosis, and infiltration of leucocytes and macrophages between the epithelial cells. Ionoregulatory parameters, which showed minor changes in fish in the initial mixing zone, did not allow explanation of acute fish mortality. Data on the ventilation frequency and the blood hematocrit, which both increased, gave support for the hypothesis that acute fish mortality in mixing zones could be caused by respiratory dysfunction.

Some physiological parameters were measured in adult rainbow trout during a 10-day exposure to 180 pg Al,,,,, I-' in acid water (pH 4.7) with or without humic substances (10 mg I-'). The fish were acclimatized to pH 5.0 for 7 days prior to the experimental treatments.Chemical analyses revealed that, in the presence of human substances, 7&80% of the A1 was organic bound, while in the absence of humic substances most of the Al(98%) occurred in the inorganic form.A1 bound to humic substances (13G150 pg I-') did not alter the plasma NaC1-concentration, nor the haematocrit value, of rainbow trout during an exposure period of 10 days. This contrasts with the high death rate obtained within 2-3 days when most of the A1 (175 pg I-') was in the inorganic form. The lethality was accompanied by a 25% decrease in the plasmaconcentration of NaCl and a doubling of the haematocrit value. Bulk analysis revealed that when the metal was present in inorganic forms the total Al content of the gills (75 wg A1 g-' wet weight) was I5 times higher than when it was present as bound to the humic substances. These experiments showed that the accumulation of A1 at the gills was accompanied by physiological disturbances, both being a function of the chemical speciation of Al.

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A 10-fold increase of plasma epinephrine and norepinephrine levels was evident at 46 h of Al exposure in adult rainbow trout, Oncorhynchus mykiss, which were kept for about 2.5 d at pH 5.0 with 60 μg Al/L (Ca2+ = 28 μmol/L). The change of plasma epinephrine levels was related both to the decrease of the blood pH and the decrease of the blood [Formula: see text]. We further observed decreased plasma Na+ concentrations which were accompanied by elevated levels of Cortisol in the plasma of Al-exposed fish. Exposure of fish to pH 6.8 (= control) or pH 5.0 without Al did not yield any changes in plasma Na+ concentrations, plasma Cortisol concentrations, blood pH, blood [Formula: see text], and plasma epinephrine, norepinephrine, and dopamine levels. The release of plasma catecholamines associated with blood acidosis and hypoxia is suggested to be an important factor in maintaining erythrocytic pH to protect the haemoglobin oxygen carrying capacity in fish exposed to low pH and Al.