Interaction between phenanthrene and zinc in their toxicity to the sheepshead minnow (Cyprinodon variegatus)

Many contaminated sites contain a variety of toxicants. Risk assessment and the development of water quality criteria therefore require information on the interactive effects among toxicants in such mixtures. Interactions between metals are relatively well studied, but little is known about interactions between metals and hydrocarbons. This study investigated the interaction between phenanthrene and zinc in the sheepshead minnow Cyprinodon variegatus. Interaction studies were performed with 7-day-old minnows in 96-h bioassays with zinc and phenanthrene at a fixed ratio and with varying proportions of zinc and phenanthrene. Mixture toxicity was quantified with the toxic unit, additive index, and excess function methods. All three methods generally indicated an antagonistic interaction between phenanthrene and zinc, though the results also provide some evidence for a synergistic interaction at low toxicant levels or at specific phenanthrene-to-zinc ratios. Short-term uptake experiments were conducted to determine if the strong antagonistic interaction observed when zinc and phenanthrene were present at 50% of their LC50 values was due to effects of zinc and phenanthrene on each other's uptake. Significantly less Zn-65 uptake occurred in the presence of phenanthrene than in its absence. In contrast, zinc did not appear to affect the uptake of C-14-phenanthrene.