Enhancement of toxic effects of phenanthrene to Daphnia magna due to the presence of suspended sediment

• Immobilization of Daphnia magna caused by PHE was enhanced in the presence of SPS.
• PHE sorbed on SPS inhibited the total superoxide dismutase activity of D. magna.
• PHE sorbed on SPS (1–5 g L−1) contributed to 37–63% of the immobilization of D. magna.
• The bioavailable fraction of PHE sorbed on SPS to D. magna ranged from 10% to 23%.
• HOCs sorbed on SPS should be considered in the establishment of water quality standard.

In the present work, the influences of suspended sediment (SPS) on the toxic effects of phenanthrene (PHE), one kind of polycyclic aromatic hydrocarbons, to Daphnia magna was studied using a dialysis bag simulation system, which equalized the freely dissolved concentration of PHE between outside the dialysis bag in the presence of SPS and inside the dialysis bag in the absence of SPS. The immobilization and total superoxide dismutase (T-SOD) activity of Daphnia magna caused by PHE (0–0.8 mg L−1) were investigated under the influence of different SPS concentrations (0, 1, 3, 5 g L−1) during a 96 h-exposure. The results showed that, compared to the absence of SPS, the presence of SPS (1–5 g L−1) increased the immobilization of Daphnia magna by 1.6–2.7 times when the freely dissolved concentration of PHE was identical in both systems. The inhibition of T-SOD activity of Daphnia magna by PHE was significantly greater in the presence of SPS than in the absence of SPS (p < 0.01). This infers that the PHE sorbed on SPS might be bioavailable and enhanced the toxic effect of PHE to Daphnia magna. The bioavailable fraction of PHE sorbed on SPS ranged from 10.1% to 22.7%, and the contribution of PHE sorbed on SPS to the immobilization caused by total PHE in the exposure system increased with SPS concentration, with the contribution ratio increasing from 36.7% to 57.7% when SPS concentration increased from 1 to 5 g L−1. This study suggests that only considering the concentrations of hydrophobic organic compounds in the water phase may underestimate their toxicity; and the hydrophobic organic compounds sorbed on SPS should not be ignored in assessment of water quality and the establishment of water quality standard in the future.

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