PHYSICOCHEMICAL CHANGES OF ALUMINUM IN MIXING ZONES - MORTALITY AND PHYSIOLOGICAL DISTURBANCES IN BROWN TROUT (SALMO-TRUTTA L)

A standardized laboratory setup, simulating field mixing zones that or iginate at the confluence of limed rivers with acidic, aluminium-rich, tributaries, has been developed. Detailed analyses of the chemical sp eciation of aluminium (Al) in relation to the biological response of b rown trout were performed to identify the mechanism of unexpected high fish mortality in the above-mentioned mixing zones with pH levels abo ve 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 + 6.8 mu mol Al/L), although the Al concentration in the mi xing zone was less (2.8 mu mol Al/L) at a nonharmful pH level (pH 6.4) . Chemical speciation and ultrafiltration studies demonstrated that th e transformation of low molecular weight Al (<10 kD) into high-molecul ar weight Al (>10 kD), defined as Al polymerization, could better be r elated to the toxic response of fish, than the total Al concentration. The aging of polymerized Al forms (for 480 s) resulted in reduced fis h mortality and less pronounced physiological stress. Brown trout in t he initial mixing zone showed significantly increased plasma glucose a nd 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 apo ptosis, and infiltration of leucocytes and macrophages between the epi thelial cells. Ionoregulatory parameters, which showed minor changes i n fish in the initial mixing zone, did not allow explanation of acute fish mortality. Data on the ventilation frequency and the blood hemato crit, which both increased, gave support for the hypothesis that acute fish mortality in mixing zones could be caused by respiratory dysfunc tion.