Evaluation of the mayfly Ephoron virgo for European sediment toxicity assessment

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The mayfly Ephoron virgo plays a key role in the ecological recovery in large European rivers. The larvae spend their life in the sediment and are therefore exposed to contaminants in various ways. In this study, the use of E. virgo as a test species for whole sediment bioassays was evaluated. The sensitivity was compared with Hexagenia limbata (a North American mayfly recommended for standard sediment testing). Lead (Pb) was used as a model contaminant. Seven Pb concentrations (range, 100–3,200 μg g−1) were spiked to uncontaminated natural sediment. Prior to the bioassays, spiked sediments were equilibrated for 35 days to ensure the partitioning of Pb among overlying water, pore water and sediment. E. virgo and H. limbata were exposed to Pb-contaminated sediments for 21 days in the test jars containing 250 mg sediment (i.e. 4 cm deep; 250 ml test medium). The exposure was performed as a semi-static system of which overlying water was renewed twice per week. Pb concentrations in pore water and sediment were measured at the beginning and the end of the assays. Pb bioavailability in the sediment was assessed using the equilibrium partitioning model based on acid volatile sulphide (AVS) and simultaneously extracted metal (SEM). A clear dose–response relationship between E. virgo survival and Pb toxicity/bioavailability in the sediment was obtained, with the lowest observed effect concentration of 2,071 μg Pb g−1 treatment [(SEMPb–AVS)/fraction of organic carbon (OC) = 168.9 μmol g−1 OC]. The results also confirmed the SEM–AVS model as no Pb toxicity occurred at (SEMPb–AVS) ≤0 μmol g−1. Comparing the responses of the two mayfly species exposed to the same Pb-contaminated sediment, E. virgo showed a higher sensitivity. The difference in sensitivity to Pb of the two mayflies could be related to intrinsic species sensitivity. Some factors which might influence sensitivity, i.e. the developmental stage and homogeneity of the larval age at exposure, should also be considered. Comprehensive data for the risk assessment of European sediments may be obtained from sediment bioassays with indigenous species. In this study, a chronic sediment toxicity assay using larvae of E. virgo has been demonstrated. Considering the sensitivity and the need to incorporate more ecologically relevant data, it can be concluded that the 21-day whole sediment toxicity test with E. virgo is a good bioassay for testing sediment toxicity.