INFLUENCE OF FULVIC ACID FRACTIONATION BY METAL HYDROXIDES ON DAPHNIA MAGNA COPPER TOXICITY

Free copper (Cu²⁺) is a well-known contributor to heavy metal toxicity in aquatic systems, particularly streams impacted by acid mine drainage (AMD). The concentration and bioavailability of Cu²⁺ is influenced by aqueous complexation with various ligands that include humic (HA) and fulvic acids (FA) being especially important. The FAs used in this study, isolated from three alpine watersheds in Central Colorado, showed variability in the spectroscopic properties of SUVA₂₅₄ (1.32 – 4.88 L mg^-1 m^-1) and fluorescence index (1.25 – 1.58). SUVA₂₅₄ was used to correlate FA spectroscopic properties to copper acute EC₅₀ values for Daphnia magna. Some FAs used for toxicity testing had undergone chemical fractionation, a result of partial removal by adsorption to hydrous metal oxides in either the stream or in laboratory experiments. The FAs remaining in solution were found to have a less protective effect on copper toxicity than the FAs isolated from aquatic systems without metal oxides present. Measured EC₅₀ varied widely (19 – 146 µg Cu/L), primarily due to chemical fractionation with an additional effect of watershed source and seasonality. A comparison of measured and computed EC₅₀ showed that the Biotic Ligand Model (BLM) accurately predicts EC₅₀, within an allowable factor of 2, for unfractionated FA. However, the BLM tends to overestimate EC₅₀ for FA that has undergone fractionation. Moderate adjustments to the BLM resulted in slight improvement in EC₅₀ and SUVA₂₅₄ associations. Variation in FA-Cu binding affinity may be a significant factor influencing copper toxicity in mining-impacted aquatic systems and may require continued adjustments in geochemical and toxicological modeling programs such as the BLM.