THE TOXIC MIXING ZONE OF NEUTRAL AND ACIDIC RIVER WATER - ACUTE ALUMINUM TOXICITY IN BROWN TROUT (SALMO-TRUTTA L)

Mixing of acid river water containing aluminium (pH 5.1, Al 345 mu g.l (-1)) with neutral water of a lake (pH 7.0, Al 73 mu g.l(-1)) resulted in water (pH 6.4, Al 245 mu g.l(-1)) with a pH (6.4) and Al concentra tion (245 mu g.l(-1)) expected to have low toxicity to fish on the bas is of current Al toxicity models. However, under semi-field conditions the freshly mixed water (a few sec. after mixing) proved to be highly toxic to brown trout. The fish were exposed to the water at different places along a 30 m channel. At the beginning of the channel acid and neutral water were continuously mixed; the mixed water left the chann el after 340 sec. The cells of the gills showed a highly increased rat e of cell death by apoptosis and necrosis. Intercellular spaces were e nlarged, and many leucocytes penetrated in these spaces. Mucus release was stimulated to depletion. Plasma chloride levels were hardly affec ted. There was a clear gradient in the deleterious effects on the fish along the channel. The fish at the beginning of the channel (about 12 sec. after mixing of the water), were severely affected, whereas the fish kept at the end of the channel (340 sec. after mixing) were only mildly affected In the natural situation fish will relatively quickly pass through a mixing zone. In our study we therefore focused on the e ffects on fish after a 60 min exposure to a mixing zone (5 sec after m ixing), with subsequent recovery in a region downstream of the conflue nce and in neutral water with low Al. The recovery in the downstream a rea (at the end of the channel, i.e. 5 min after mixing) was clearly h ampered when compared to the recovery in neutral water with low alumin ium Thus, a short exposure to the toxic mixing zone followed by a stay in water downstream of this zone, as may occur in nature, is detrimen tal to migrating trout. We conclude that freshly mixed acid and neutra l water contain toxic components during the first seconds to minutes a fter mixing that can not be explained by current models on aluminium t oxicity.