Bioavailability and toxicity of metals in sediment were assessed in three water bodies in the State of São Paulo, SE Brazil, in one of the first investigations of the topic in the country. Weakly bound metals in bulk sediment samples taken from a highly-polluted reservoir, Billings, São Paulo, exhibited enrichment factors from 2.4 to 30 fold, compared to two other water bodies in the study, Barra Bonita Reservoir and Diogo Lake. High mortality rates occurred when Hyalella azteca and Ceriodaphnia dubia were exposed to Billings reservoir sediment. After forty-eight days of sediment oxidation, laboratory microcosms showed increases in the weakly-bound fraction of metals in sediments from Billings Reservoir (all metals), Diogo Lake (all metals except Zinc) and Barra Bonita Reservoir (Cadmium and Lead only). Chironomus xanthus was not sensitive to toxicity in sediment from any of the three environments. Ceriodaphnia dubia was sensitive to chronic toxicity in bulk field sediments from all three environments, and showed an increase in mortality rate in tests with oxidized sediment. Billings sediment, with or without aeration, caused 100 percent mortality, while in contact with Barra Bonita field sediment, a mortality of 10 percent of the adult organisms occurred, increasing up to 80 percent in post-aeration sediments. In Diogo Lake sediment, 30 percent mortality was observed in field or pre-aeration sediment rising to up to 60 percent in post aeration sediments. The two factors caused by oxidation of reduced substances in sediments, a fall in pH in the water and mobilization of bound metal, are invoked to explain this response.

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