Role of Organic Complexation in the Toxicity of Aluminum to Rana pipiens Embryos and Bufo americanus Tadpoles

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Abstract
This study investigated the influence of naturally occurring organic compounds on the survival of amphibian embyryos and larvae in acidic, aluminum contaminated water. A series of laboratory tests were conducted which measured aluminum toxicity in artificial soft water (ASW) diluted with varying amounts of pond water which contained dissolved organic compounds. At pH 4.5 for tadpoles of Bufo americanus and pH 4.8 for embryos of Rana pipiens, the organic compounds (DOC 5.7 to 16.2 mg .cntdot. L-1) complexed the aluminum and thus increased the LC50 for total aluminum by more than two fold. At pH 4.2, 100% ASW and the four dilutions of pond water were toxic to R. pipiens embryos (75-96% mortality). In contrast to higher pH''s, aluminum ameliorated acid toxicity which resulted in a 26% reduction in mortality in 100% ASW at 250 mg .cntdot. L-1 total aluminum. Although the organic compounds in the pond water also complexed aluminum at pH 4.2, protective levels of labile aluminum (.gtoreq. 180 .mu.g .cntdot. L-1) were still present at 450 .MU.g. L-1 total aluminum. However, at a given level of labile aluminum in pH 4.2 water, pond water was more toxic to embryos than ASW. At pH 4.2, pond water without added Al was also significantly more toxic to embryos than 100% ASW with 0 .mu.g. L-1 Al. Toxicity tests using purified humic and fulvic acid suggested that these compounds may be the toxic agents in high dissolved organic carbon (DOC) pond water.
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