Assessment of the toxicity of River Po sediments with Ceriodaphnia dubia

A recent interdisciplinary project aimed at the assessment of the quality o f the River Po focusing primarily on the bed sediments of the principal Ita lian river. In such a moderate/low polluted environment, the identification of sites at risk using a number of toxicological tools was considered of g reat interest. The present study examines in detail the information which w as obtained from two of the toxicological approaches adopted in that projec t, that is, the 7-day whole-sediment test with Ceriodaphnia dubia and the a cute toxicity test (48 h) with the same crustacean conducted on an organic- solvent extract of the sediment fine material (< 63 mu m). These toxicity t ests were applied to composite sediment samples collected in summer and win ter low-flow conditions, downstream from the main tributaries of the River Po. Summer and winter sediments seemed to be characterized by different qua lities, as suggested by their oxygen demands and C/N ratios, whereas they w ere 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 s ediments cause chronic toxic stress, whereas the winter sediments enhance r eproduction and growth. Apart from these seasonal differences, it was found that the biomass production (reproduction + somatic growth) of C. dubia wa s negatively correlated to the fraction of sediment organic carbon contribu ted by fine material ( < 63 mu 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 rel atively independent from seasons and substantially stable, particularly alo ng the larger potamon section. The concurrent examination of biomass produc tion and sediment TUs confirmed the sites at risk and demonstrated that the biomass production of C. dubia is inhibited by the increase of sediment TU s. This examination also demonstrated that some sediments, particularly of the winter survey, have high toxic potentials which, however, were not effe ctive. Apparently this was a result of different source and quality of sedi ment organic matter, which may affect either the bioavailability of chemica ls or the nutritional value of fine particles. (C) 2000 Elsevier Science B. V. All rights reserved.