Water pH and aluminum chemistry in the gill micro-environment of rainbow trout during acid and aluminum exposures

Rainbow trout (Salmo gairdneri) were exposed to acidic soft water (pH_in4.2-6.3) in the presence (93 μg·l^-1) or absence of Al. Fish were fitted with latex masks and opercular catheters to measure ventilation {Mathematical expression}, pH changes at the gills, O₂ consumption {Mathematical expression}, ammonia excretion {Mathematical expression}, and Al extraction. During 2-3-h exposures, {Mathematical expression} was generally higher in Al-exposed fish over the pH_in range 4.7-6.3. Alkalinization of expired water was about 0.3 pH units less in Al-exposed fish than in acid-only exposed fish at pH_in 4.5-5.2, an effect attributable to both increased {Mathematical expression} and to buffering by Al. During 44-h exposures to pH_in 5.2 and 4.8 plus Al, {Mathematical expression} increased greatly and expired water pH (pH_ex) decreased with time. There was a small increase in {Mathematical expression} over 44 h at pH 4.4 plus Al, and no changes in pH_ex. In contrast, during 44-h exposures to pH 5.2, 4.8, and 4.4 in the absence of Al, such changes were much smaller or absent. During both short- and longerterm exposures, measured Al accumulation on the gills was only 5-18% of that calculated from cumulative Al extraction from the water, suggesting considerable sloughing of Al. In free-swimming trout, gill Al accumulation was greatest during exposure (2h) to Al at pH 5.2, lower at pH 4.8, and least at pH 4.4 and 4.0. Our results suggest that Al deposition occurs at the gills, causing respiratory and ionoregulatory toxicity, because the pH in the branchial micro-environment is raised above that in the acidic inspired soft water. Higher pH at fish gills may result in Al precipitation due to loss of solubility, or Al accumulation because of shifts in Al species to Al-hydroxide forms which more readily adsorb to the gills.