Mixtures of metals and hydrocarbons elicit complex responses by a benthic invertebrate community

Abstract

A microcosm experiment was used to examine the effects of co-contamination with environmentally realistic but relatively low concentrations (expected to cause occasional to frequent adverse effects) of diesel fuel and metals (Cu, Cd, Hg, Cr, and Pb) on a saltmarsh benthic invertebrate community. After 30 days, exposure to metals did not influence abundances of major meiofaunal taxa (nematodes, ostracods, total copepods, nauplii, or chironomid larvae) or individual copepod species. Diesel exposure did not influence nematode or total copepod abundances, but significantly decreased abundances of ostracods, nauplii, and chironomids. For all taxa except copepods, the metals+diesel treatment yielded results similar to the effects of diesel alone. Although total copepod abundances were not significantly influenced by metals- or diesel-only treatments, abundances were significantly reduced in the metals+diesel treatment, a response that appeared consistent with a non-additive toxicological synergism between metals and diesel. Responses of copepod species to the diesel-only treatment were varied; species could be categorized as ‘diesel-sensitive’ (declined in abundance, presumably due to direct (toxic) effects), or ‘diesel-resistant’ (abundance either unchanged, or increased due to indirect (ecological) effects). In the metals+diesel treatment, the effects on ‘diesel-sensitive’ species were similar to those observed in the diesel-only treatment. However, abundances of ‘diesel-resistant’ species declined in the metals+diesel treatment, suggesting that metals interfere with the indirect effects that lead to enhanced abundances of some species and/or that metal+diesel combinations are uniquely toxic. Collectively, however, responses of most individual copepod species to metals+diesel suggested an additive toxicological action, and thus differed from the apparent synergism observed for total copepod abundance. The presence of diesel enhanced the retention of metals in sediments, which may have important toxicological implications. We conclude that, at relatively low concentrations of metal and diesel fuel co-contamination, both direct (toxic) and indirect effects influence faunal abundance; thus, reductions in abundance of major taxa may give the appearance of toxicological synergisms that are not manifested at the species level.