We studied the influence of heavy metals, organic matter (subdivided into humic compounds and not humified organic matter which represents all organic matter fractions except for humic and fulvic acids), total sulphur and nitrogen content on the toxicity of sediments from the Mar Piccolo, a semienclosed basin located near the city of Taranto (Southern Italy). In sediments, we found high level of total lead and zinc concentration but a very low "available metal concentration", that consists in the metal fraction extractable into MgCl2 solution at pH 7, for all heavy metals. Moreover, cluster analysis pointed to a common source of organic matter, sulphur, nitrogen, lead, copper, zinc and mercury. As regards the ecotoxicological results, sediments were found to be characterized by a number of toxic units (T.U.) ranging from 1305 to 3693 T.U., indicating a medium/high toxicity level. Multiple linear regression analysis carried out using available lead, nickel and manganese concentrations, as well as sulphur and non-humic carbon as independent variables and toxicity units as the dependent variable, accounted for a good percentage of the total variance of data (R2=91%). Nevertheless, sulphur and not humic carbon were the only significant components (p-value: 0.022 and 0.023, respectively). © 2004 Elsevier B.V. All rights reserved.

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