Decreased toxicity of Al to juvenile Atlantic salmon (Salmo salar) in acidic soft water containing natural organic matter: A test of the free-ion model

Toxicological experiments were designed to test the predictions of the free-ion model (FIM) in a soft, acidic water containing Al and a natural fulvic acid (FA). Juvenile Atlantic salmon (Salmo salar) were exposed to Al in either inorganic Or organic acidic soft waters, at pH 5.0, and 50% lethal concentrations (LC50) values were determined for both exposure solutions. Contrary to the predictions of the FIM, the LC50s, expressed as inorganic Al (Al-i), were significantly higher (p < 0.05) in the organic solution (6.1-8.0 mu M Al-i) than in the inorganic medium (2.9-4.0 mu M Al-i). Preexposure of the fish to a 10-mg/L FA solution at pH 5.0. without Al, did not protect the animals front a subsequent Al challenge in solutions without FA. A regression model related mortality with Al-i and total organic carbon (TOC) and was highly significant (proportion mortality [arcsine] = -0.215 + [0.81]log[Al-i] - [0.44]TOC; p < 0.0001, R-2 = 0.71). We conclude that natural FA may play an independent protective role in solutions containing Al, in addition to that of decreasing Al toxicity by complexation of the metal and reducing the free Al3+ concentration.