Bioavailability and acute toxicity of copper to rainbow trout (Oncorhynchus mykiss) in the presence of organic acids simulating natural dissolved organic carbon

Copper bioavailability and toxicity to early life stage rainbow trout (Onco rhynchus mykiss) were evaluated by laboratory toxicity testing performed us ing organic acid mixtures. Geochemical modeling was used to design exposure solutions that simulate dissolved organic carbon (DOC) of a natural aquati c system and to determine the fractions of total Cu present as inorganic sp ecies (e.g., Cu2+) and as individual Cu-organic complexes. Failure time mod eling indicated that mortality was best predicted by a combination of total inorganic Cu and distinct Cu-organic complexes. The Cu-organic complexes t hat contributed to toxicity are characterized as low-affinity Cu-ligands, a nd our results support the hypothesis that Cu toxicity in nature is a funct ion of the binding characteristics of individual ligands. Estimates of time -independent median lethal concentration thresholds determined at widely va rying equivalent concentrations of DOC (0-16 mg/L) were constant (7.9-8.6 m u g Cu/L) when modeled using the sum of inorganic Cu and Cu bound to the tw o low-affinity ligands as predictors of toxicity. Our results indicate that Cu bound to organic complexes may be available to fish and that acute toxi city of Cu is determined by the binding affinities of specific DOC componen ts relative to Cu-binding affinity of fish gill.