Assessment of the toxicity of River Po sediments with Ceriodaphnia dubia

A recent interdisciplinary project aimed at the assessment of the quality of the River Po focusing primarily on the bed sediments of the principal Italian river. In such a moderate/low polluted environment, the identification of sites at risk using a number of toxicological tools was considered of great interest. The present study examines in detail the information which was obtained from two of the toxicological approaches adopted in that project, that is, the 7-day whole-sediment test with Ceriodaphnia dubia and the acute toxicity test (48 h) with the same crustacean conducted on an organic-solvent extract of the sediment fine material (<63 ?m). These toxicity tests were applied to composite sediment samples collected in summer and winter low-flow conditions, downstream from the main tributaries of the River Po. Summer and winter sediments seemed to be characterized by different qualities, as suggested by their oxygen demands and C/N ratios, whereas they were indistinguishable for the contents of organic carbon and fine material. For some reaches of the River Po, the 7-day tests showed that the summer sediments cause chronic toxic stress, whereas the winter sediments enhance reproduction and growth. Apart from these seasonal differences, it was found that the biomass production (reproduction+somatic growth) of C. dubia was negatively correlated to the fraction of sediment organic carbon contributed by fine material (<63 ?m). The results of the extract tests showed that the toxic potentials of sediments vary along the River Po, identifying some critical sites. In contrast with whole-sediment results, it was found that the toxic potentials, reported as sediment toxic units (TUs), are relatively independent from seasons and substantially stable, particularly along the larger potamon section. The concurrent examination of biomass production and sediment TUs confirmed the sites at risk and demonstrated that the biomass production of C. dubia is inhibited by the increase of sediment TUs. This examination also demonstrated that some sediments, particularly of the winter survey, have high toxic potentials which, however, were not effective. Apparently this was a result of different source and quality of sediment organic matter, which may affect either the bioavailability of chemicals or the nutritional value of fine particles. Copyright (C) 2000 Elsevier Science B.V.