Influence of Organic Matter Fractionation on Aquatic Copper Toxicity In Mineralized Environments

Chemical speciation of dissolved metals influences their potential toxicity to aquatic biota. The presence of dissolved organic matter (DOM) is known to decrease copper toxicity in aquatic systems due to copper binding to DOM. However, in iron-rich environments the protective effect of DOM on copper toxicity is reduced. We investigated the ability of DOM isolated from different sources to decrease acute copper toxicity to Ceriodaphnia dubia (water flea). The DOM was isolated from filtered stream water using XAD-8 columns, followed by saturation with H⁺ and freeze drying. We conducted acute toxicity tests in laboratory-prepared EPA moderately-hard water amended with 6 mg/L DOM that was isolated from (1) a stream receiving acidic iron- and aluminum-rich drainage, (2) an adjacent stream not impacted by iron- and aluminum-rich drainage, and (3) Suwannee River fulvic acid. Our results show that the DOM isolated from a stream receiving acidic iron- and aluminum-rich drainage was 3 times less effective at reducing copper toxicity than was DOM isolated from the other two sources. In contrast, organic matter isolated from sediment in the iron- and aluminum-rich stream was 1.5 times more effective at reducing copper toxicity than was DOM isolated from the two non-impacted sources. These findings demonstrate that fractionation of organic matter between the dissolved and sediment phases in iron- and aluminum-rich systems can result in more bioavailable dissolved copper and greater potential for copper toxicity to aquatic organisms. This is because DOM with greater affinity for metal binding tends to be preferentially sorbed to the sediment phases in iron- and aluminum-rich systems.