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 bet ween metals are relatively well studied, but little is known about interact ions between metals and hydrocarbons. This study investigated the interacti on between phenanthrene and zinc in the sheepshead minnow Cyprinodon varieg atus. 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 o f zinc and phenanthrene. Mixture toxicity was quantified with the toxic uni t, additive index, and excess function methods. All three methods generally indicated an antagonistic interaction between phenanthrene and zinc, thoug h the results also provide some evidence for a synergistic interaction at l ow toxicant levels or at specific phenanthrene-to-zinc ratios. Short-term u ptake experiments were conducted to determine if the strong antagonistic in teraction 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 upt ake. Significantly less Zn-65 uptake occurred in the presence of phenanthre ne than in its absence. In contrast, zinc did not appear to affect the upta ke of C-14-phenanthrene.