While it is likely that ENPs may occur together with other contaminants in nature, the combined effects of exposure to both ENPs and environmental contaminants are not studied sufficiently. In this study, we investigated the acute and sublethal toxicity of PVP coated silver nanoparticles (AgNP) and ionic silver (Ag⁺; administered as AgNO₃) to the marine copepod Calanus finmarchicus. We further studied effects of single exposures to AgNPs (nominal concentrations: low 15 μg L^-1 NPL, high 150 μg L^-1 NPH) or Ag⁺ (60 μg L^-1), and effects of co-exposure to AgNPs, Ag⁺ and the water-soluble fraction (WSF; 100 μg L^-1) of a crude oil (AgNP + WSF; Ag⁺+WSF). The gene expression and the activity of antioxidant defense enzymes SOD, CAT and GST, as well as the gene expression of HSP90 and CYP330A1 were determined as sublethal endpoints. Results show that Ag⁺ was more acutely toxic compared to AgNPs, with 96 h LC₅₀ concentrations of 403 μg L^-1 for AgNPs, and 147 μg L^-1 for Ag⁺. Organismal uptake of Ag following exposure was similar for AgNP and Ag⁺, and was not significantly different when co-exposed to WSF. Exposure to AgNPs alone caused increases in gene expressions of GST and SOD, whereas WSF exposure caused an induction in SOD. Responses in enzyme activities were generally low, with significant effects observed only on SOD activity in NPL and WSF exposures and on GST activity in NPL and NPH exposures. Combined AgNP and WSF exposures caused slightly altered responses in expression of SOD, GST and CYP330A1 genes compared to the single exposures of either AgNPs or WSF. However, there was no clear pattern of cumulative effects caused by co-exposures of AgNPs and WSF. The present study indicates that the exposure to AgNPs, Ag⁺, and to a lesser degree WSF cause an oxidative stress response in C. finmarchicus, which was slightly, but mostly not significantly altered in combined exposures. This indicated that the combined effects between Ag and WSF are relatively limited, at least with regard to oxidative stress.

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